Article: Connectionless-mode Network Service (CLNS) or simply Connectionless Network Service is an OSI Network Layer datagram service that does not require a circuit to be established before data is transmitted, and routes messages to their destinations independently of any other messages. As such it is a "best-effort" rather than a "reliable" delivery service. CLNS is not an Internet service, but provides capabilities in an OSI network environment similar to those provided by the Internet Protocol (IP) and the User Datagram Protocol (UDP).
Connectionless-mode Network Protocol (CLNP)
Is an OSI protocol deployment. CLNS is the service provided by the Connectionless-mode Network Protocol (CLNP). CLNP is widely used in many telecommunications networks around the world because IS-IS (an OSI routing protocol) is mandated by the ITU-T as the protocol for management of Synchronous Digital Hierarchy (SDH) elements. From August 1990 to April 1995 the NSFNET backbone supported CLNP in addition to TCP/IP. However, CLNP usage remained low compared to TCP/IP.
Transport Protocol Class 4 (TP4) in conjunction with CLNS
CLNS is used by ISO Transport Protocol Class 4 (TP4), one of the five transport layer protocols in the OSI suite. TP4 offers error recovery, performs segmentation and reassembly, and supplies multiplexing and demultiplexing of data streams over a single virtual circuit. TP4 sequences PDUs and retransmits them or re-initiates the connection if an excessive number are unacknowledged. TP4 provides reliable transport service and functions with either connection-oriented or connectionless network service. TP4 is the most commonly used of all the OSI transport protocols and is similar to the Transmission Control Protocol (TCP) in the TCP/IP protocol suite.
Protocols providing CLNS
Several protocols provide the CLNS service:
Connectionless-mode Network Protocol (CLNP), as specified in ITU-T Recommendation X.233.
End System-to-Intermediate System (ES-IS), a routing exchange protocol for use in conjunction with the protocol for providing the CLNS (ISO 9542).
Intermediate System-to-Intermediate System (IS-IS), an intradomain routing exchange protocol used in both the OSI and Internet environments (ISO 10589 and RFC 1142).
Signalling Connection Control Part (SCCP), as specified in ITU-T Recommendation Q.711 is a Signaling System 7 protocol.
• OSPF supports only IP, IS-IS supports both IP and CLNS.
• IS-IS does not require IP connectivity between routers to share routing
information. Updates are sent via CLNS instead of IP.
• In OSPF, interfaces belong to areas. In IS-IS, the entire router
belongs to an area.
• An IS-IS router belongs to only one Level-2 area, which results in less
LSP traffic. IS-IS is thus more efficient and scalable than OSPF, and
supports more routers per area.
• There is no Area 0 backbone area for IS-IS. The IS-IS backbone is a
contiguous group of Level 1-2 and Level 2 routers.
• IS-IS does not elect a backup DIS. Additionally, DIS election is
preemptive.
• On broadcast networks, even with an elected DIS, IS-IS routers still
form adjacencies with all other routers. In OSPF, routers will only
form adjacencies with the DR and BDR on broadcast links.
• IS-IS uses an arbitrary cost metric. OSPF’s cost metric is based on the
bandwidth of the link.
• IS-IS provides far more granular control of link-state and SPF timers
than OSPF See also OSI model TCP/IP model
X.25 protocol suite, an OSI Connection Oriented Network Service (CONS)
References External links
What is CLNS? - a brief introduction by Ivan Pepelnjak |
Source: Pages Home
Tuesday 28 February 2012
A Tour with Network
A Tour with Network
Posted by John at 12:37:00 0 comments Email This BlogThis! Share to Twitter Share to Facebook Share to Pinterest
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Wednesday 21 December 2011
Routing CCNP NOTES
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
1 ___________________________________________
Cisco CCNP Routing Study Guide
v1.12 © 2007 ________________________________________________ Aaron Balchunas aaron@routeralley.com http://www.routeralley.com ________________________________________________ Foreword:
This study guide is intended to provide those pursuing the CCNP
certification with a framework of what concepts need to be studied. This is
not a comprehensive document containing all the secrets of the CCNP
Switching exam, nor is it a “braindump” of questions and answers.
This document is freely given, and can be freely distributed. However, the
contents of this document cannot be altered, without my written consent.
Nor can this document be sold or published without my expressed consent.
I sincerely hope that this document provides some assistance and clarity in
your studies. ________________________________________________
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
2 Table of Contents
Part I – Addressing
Section 1 IPv4 Addressing
Section 2 IPv6 Addressing
Section 3 TCP & UDP
Part II – Basic Routing Concepts
Section 4 The Routing Table
Section 5 Classful vs. Classless Routing
Section 6 Static vs. Dynamic Routing
Section 7 Configuring Static Routes
Section 8 Default Routing
Part III – Dynamic Routing Protocols
Section 9 RIP v1 & v2
Section 10 IGRP Section 11 EIGRP Section 12 OSPF Section 13 IS-IS Section 14 BGP
Part IV– Advanced Routing Functions
Section 15 Route Redistribution
Section 16 Access Control Lists
Section 17 Route Filtering and Route-Maps
Section 18 Multicast
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
3 ________________________________________________ Part I Addressing ________________________________________________
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
4 Section 1
- IPv4 Addressing and Subnetting -
Hardware Addressing
The hardware address is used by devices to communicate on the local
network. Hardware addressing is a function of the data-link layer of the OSI
model (Layer-2).
The hardware address for Ethernet networks is the MAC address, a 48-bit
hexadecimal address that is usually hard-coded on the network card. In
theory, this means the MAC address cannot be altered; however, the MAC
address is often stored in flash on the NIC, and thus can be changed with
special utilities.
MAC addresses can be represented in two formats (either notation is
acceptable): 00:43:AB:F2:32:13 0043.ABF2.3213
The MAC address has one shortcoming – it contains no hierarchy. There is
no mechanism to create boundaries between networks.
Instead, the first six hexadecimal digits of a MAC identify the manufacturer
of the network card (referred to as the OUI (Organizational Unique
Identifier)), and the last 6 digits identify the host device (referred to as the
host ID). Still, there is no way to distinguish one network from another.
Image the difficulties this poses. If only hardware addressing existed, all
devices would technically be on the same network. Modern internetwork
systems like the Internet could not exist, as there would be no way to
separate my network from your network.
Furthermore, imagine if the entire Internet was a purely switched, data-link
layer environment. Switches, as a rule, forward broadcasts out all ports.
Guesstimating that there are billions devices on the Internet, with each
device sending out a broadcast on average every few seconds, the resulting
broadcast storms would be devastating. The Internet would simply collapse.
The need for logical addressing, and routers, became apparent.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
5 Logical Addressing
Logical addressing is a function of the network layer of the OSI Model
(Layer-3).
Logical addresses, unlike hardware addresses, provide a hierarchical
structure to separate networks. A logical address identifies not only a unique
Host ID, but also the network that host belongs to. Additionally, logical
addresses are rarely hard-coded onto hosts, and can be changed freely.
Two common logical addressing protocols are IPX (Internetwork Packet
Exchange) and IP (Internet Protocol). IPX was predominantly used on
Novell networks, but is mostly deprecated. IP is the most widely-used
logical address today. Internet Protocol (IP)
IP was developed by the Department of Defense (DoD) during the late
1970’s. It was included in a group of protocols that became known as the
TCP/IP protocol suite.
The DoD developed their own networking model to organize and define the
TCP/IP protocol suite. This became known as the DoD Model, and consists
of four layers:
OSI Model DoD Model Example Protocols
7 Application 6 Presentation 5 Session
4 Application FTP, HTTP, SMTP
4 Transport 3 Host-to-Host TCP, UDP
3 Network 2 Internet IP
2 Data-link 1 Physical
1 Network Access Ethernet
IP provides two core functions:
• Logical addressing of hosts
• Routing of packets between networks.
IP has undergone several revisions. IP Version 4 (IPv4) is currently in
widespread deployment, but will eventually be replaced with IP Version 6
(IPv6). This guide will concentrate on IPv4, and IPv6 will be covered
extensively in a separate guide.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
6 IPv4 Addressing
One of IP’s core functions is to provide logical addressing for hosts. An IP
address provides a hierarchical structure to separate networks. Consider the
following address as an example:
158.80.164.3
An IP address is separated into four octets:
First Octet
Second Octet Third Octet Fourth Octet
158 .80 .164 .3
Each octet is 8 bits long, resulting in a 32-bit IP address. A computer
understands an IP address in its binary form; the above address in binary
would look as follows:
First Octet
Second Octet Third Octet Fourth Octet
10011110 .01010000 .10100100 .00000011
Part of the above IP address identifies the network. The other part of the
address identifies the host. A subnet mask helps make this distinction.
Consider the following: 158.80.164.3 255.255.0.0
The above IP address has a subnet mask of 255.255.0.0. The subnet mask
follows two rules:
• If a binary bit is set to a 1 (or on) in a subnet mask, the corresponding
bit in the address identifies the network.
• If a binary bit is set to a 0 (or off) in a subnet mask, the corresponding
bit in the address identifies the host.
Looking at the above address and subnet mask in binary:
Address: 10011110.01010000.10100100.00000011 Subnet Mask: 11111111.11111111.00000000.00000000
The first 16 bits of the subnet mask are set to 1. Thus, the first 16 bits of the
address (158.80) identify the network. The last 16 bits of the subnet mask are
set to 0. Thus, the last 16 bits of the address (164.3) identify the unique host
on that network.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
7 IPv4 Addressing (continued)
Hosts on the same logical network will have identical network addresses,
and can communicate freely. For example, the following two hosts are on
the same network:
Host A: 158.80.164.100 255.255.0.0
Host B: 158.80.164.101 255.255.0.0
Both share the same network address (158.80), which is determined by the
255.255.0.0 subnet mask. Hosts that are on different networks cannot
communicate without an intermediating device. For example:
Host A: 158.80.164.100 255.255.0.0
Host B: 158.85.164.101 255.255.0.0
The subnet mask has remained the same, but the network addresses are now
different (158.80 and 158.85 respectively). Thus, the two hosts are not on
the same network, and cannot communicate without a router between them.
Routing is the process of sending packets from one network to another.
Consider the following, trickier example:
Host A: 158.80.1.1 255.248.0.0
Host B: 158.79.1.1 255.248.0.0
The specified subnet mask is now 255.248.0.0, which doesn’t fall cleanly on
an octet boundary. To determine if these hosts are on separate networks, first
convert everything to binary:
Host A Address: 10011110.01010000.00000001.00000001
Host B Address: 10011110.01001111.00000001.00000001
Subnet Mask: 11111111.11111000.00000000.00000000
Remember, the 1 (or on) bits in the subnet mask identify the network portion
of the address. In this example, the first 13 bits (the 8 bits of the first octet,
and the first 5 bits of the second octet) identify the network. Looking at only
the first 13 bits of each address:
Host A Address: 10011110.01010
Host B Address: 10011110.01001
Clearly, the network addresses are not identical. Thus, these two devices are
on separate networks, and require a router to communicate.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
8 IP Address Classes
The IPv4 address space has been structured into several classes. The value
of the first octet of an address determines the class of the network:
Class First Octet Range
Default Subnet Mask
Class A 1 - 127 255.0.0.0
Class B 128 - 191 255.255.0.0
Class C 192 - 223 255.255.255.0
Class A networks range from 1 to 127. The default subnet mask is
255.0.0.0; thus, by default, the first octet defines the network, and last three
octets define the host. This results in a maximum of 127 Class A networks,
with 16,777,214 hosts per network!
Example of a Class A address:
Address: 64.32.254.100 Subnet Mask: 255.0.0.0
Class B networks range from 128 to 191. The default subnet mask is
255.255.0.0; thus, by default, the first two octets define the network, and the
last two octets define the host. This results in a maximum of 16,384 Class B
networks, with 65,534 hosts per network.
Example of a Class B address:
Address: 152.4.12.195 Subnet Mask: 255.255.0.0
Class C networks range from 192 to 223. The default subnet mask is
255.255.255.0; thus, by default, the first three octets define the network, and
the last octet defines the host. This results in a maximum of 2,097,152 Class
C networks, with 254 hosts per network.
Example of a Class C address:
Address: 207.79.233.6 Subnet Mask: 255.255.255.0
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
9
CIDR (Classless Inter-Domain Routing)
Classless Inter-Domain Routing (CIDR) is simplified method of
representing a subnet mask. CIDR identifies the number of binary bits set to
a 1 (or on) in a subnet mask, preceded by a slash.
Consider the following subnet mask: 255.255.255.240
Looking at the above subnet mask in binary:
11111111.11111111.11111111.11110000
The first 28 bits of the above subnet mask are set to 1. To represent this in
CIDR notation: /28
Consider this next example:
192.168.1.1 255.255.255.0
The above address/subnet mask can be represented as follows using CIDR:
192.168.1.1 /24
Address “Classes” vs. Subnet Mask
Remember the following three rules:
• The first octet on an address dictates the class of that address.
• The subnet mask dictates what portion of an address identifies the
network, and what portion identifies the host.
• Each class has a default subnet mask.
Thus, the address 10.1.1.1 is a Class A address, and its default subnet mask
is 255.0.0.0 (or in CIDR, /8). However, it is possible to use subnet masks
other than the default, such as applying a Class B mask to a Class A address:
10.1.1.1 /16
However, this does not change the class of the above address. It remains a
Class A address, which has been subnetted using a Class B mask.
Remember, the only thing that determines the class of an IP address is the
first octet of that address. Likewise, the subnet mask is the only thing that
determines what portion of an address is the network, and which portion is
the host.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
10
Subnet and Broadcast Addresses
Two addresses have been reserved on each network for special use. Each
network must have a subnet (or network) address, and a broadcast address.
Neither of these addresses can be assigned to a host device.
The subnet address is used to identify the network itself. Routing tables
contain lists of networks, and each network is identified by its subnet
address. Subnet addresses contain all 0 bits in the host portion of the
address.
For example, the following is a subnet address: 192.168.1.0/24
The broadcast address identifies all hosts on a particular network. A packet
sent to the broadcast address will be received and processed by every device
on that network. Broadcast addresses contain all 1 bits in the host portion
of the address.
For example, the following is a broadcast address: 192.168.1.255/24
Broadcasts are one of three types of IP packets:
• Unicasts are packets sent from one host to another host
• Multicasts are packets sent from one host to a group of hosts
• Broadcasts, as stated earlier, are packets sent from one host to all
other hosts on the local network
A router, by default, will never forward a multicast or broadcast packet
from one interface to another.
A switch, be default, will forward a multicast or broadcast out every port,
except for the port that sent the multicast/broadcast.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
11 Subnetting
Subnetting is the process of creating new networks (or subnets) by stealing
bits from the host portion of a subnet mask. There is one caveat: stealing bits
from hosts creates more networks but fewer hosts per network. Thus, every
time a network is subnetted, addresses are lost.
Consider the following Class C network:
192.168.254.0
The default subnet mask for this network is 255.255.255.0. This single
network can be segmented, or subnetted, into multiple networks. For
example, assume a minimum of 10 new networks are required. Resolving
this is possible using the following magical formula:
2n – 2
The exponent ‘n’ identifies the number of bits to steal from the host portion
of the subnet mask. The default Class C mask (255.255.255.0) looks as
follows in binary: 11111111.1111111.1111111.00000000
There are a total of 24 bits set to 1, which are used to identify the network.
There are a total of 8 bits set to 0, which are used to identify the host, and
these host bits can be ‘stolen.’
Stealing bits essentially involves changing host bits (set to 0 or off) in the
subnet mask to network bits (set to 1 or on). Network bits in a subnet mask
must always be sequential, skipping bits is not allowed.
Consider the result if three bits are stolen. Using the above formula:
2n – 2 = 23 – 2 = 8 – 2 = 6 new networks created
However, a total of six new networks does not meet the original requirement
of at least 10 networks. Consider the result if four bits are stolen:
2n – 2 = 24 – 2 = 16 – 2 = 14 new networks created
A total of fourteen new networks does meet the original requirement.
Stealing four host bits results in the following new subnet mask:
11111111.11111111.11111111.11110000 = 255.255.255.240
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
12 Subnetting (continued)
In the previous example, a Class C network was subnetted to create 14 new
networks, using a subnet mask of 255.255.255.240 (or /28 in CIDR). Four
bits were stolen in the subnet mask, leaving only four bits for hosts.
To determine the number of hosts this results in, for each of the new 14
networks, the same formula can be used: 2n – 2
Consider the result if four bits are available for hosts:
2n – 2 = 24 – 2 = 16 – 2 = 14 usable hosts per network
Thus, subnetting a Class C network with a /28 mask creates fourteen new
networks, with fourteen usable hosts per network.
The “-2” Rule of Subnetting
There is a specific purpose for the ‘– 2’ portion of the 2n – 2 formula.
Previously, it was unacceptable to use an address that contained all ‘0’ or all
‘1’ bits in the network portion of the address.
However, this is no longer true on modern systems. Specifically, on Cisco
IOS devices, the following command is now enabled by default:
Router(config)# ip subnet-zero
The ip subnet-zero commands allows for the use of networks with all ‘0’ or
all ‘1’ bits in the network portion of the address. Thus, the formula for
calculating the number of new network is slightly altered, to simply 2n.
Consider if four bits are stolen for networks:
2n = 24
= 16 new networks created
However, it is never possible to assign an address with all ‘0’ or all ‘1’
bits in the host portion of the address. These are reserved for the subnet
and broadcast addresses, respectively. Thus, the formula for calculating
usable hosts is always 2n – 2.
Some have questioned whether CCNA/CCNP simulations and questions
have ip subnet-zero enabled. It is generally accepted that having this enabled
is now default behavior, and test questions should be answered accordingly.
All future examples in this guide will assume the command is enabled.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
13
Determining the ‘Range’ of Subnetted Networks
Determining the range of the newly created networks can be accomplished
using several methods. The ‘long’ method involves some binary magic.
Still looking at the example 192.168.254.0 network, which was subnetted
using a 255.255.255.240 mask:
192.168.254.0: 11000000.10101000.11111110.00000000 255.255.255.240: 11111111.11111111.11111111.11110000
Subnetting stole four bits in the fourth octet, creating a total of 16 new
networks (assuming ip subnet-zero is enabled). Looking at only the fourth
octet, the first newly created network is 0000. The second new network is
0001. Calculating all possible permutations of the four stolen bits:
Binary Decimal Binary Decimal Binary Decimal
.0000 xxxx .0 .0110 xxxx .96 .1100 xxxx .192
.0001 xxxx .16 .0111 xxxx .112 .1101 xxxx .208
.0010 xxxx .32 .1000 xxxx .128 .1110 xxxx .224
.0011 xxxx .48 .1001 xxxx .144 .1111 xxxx .240
.0100 xxxx .64 .1010 xxxx .160
.0101 xxxx .80 .1011 xxxx .176
Note that this equates to exactly 16 new networks. The decimal value
represents the first (or the subnet) address of each newly created network. To
determine the range for the hosts of the first new network:
Binary Decimal Binary Decimal Binary Decimal
.0000 0000 .0 .0000 0110 .6 .0000 1100 .12
.0000 0001 .1 .0000 0111 .7 .0000 1101 .13
.0000 0010 .2 .0000 1000 .8 .0000 1110 .14
.0000 0011 .3 .0000 1001 .9 .0000 1111 .15
.0000 0100 .4 .0000 1010 .10
.0000 0101 .5 .0000 1011 .11
The binary value has been ‘split’ to emphasize the separation of the network
bits from the host bits. The first address has all 0 bits in the host portion
(0000), and thus is the subnet address for this network. The last address has
all 1 bits in the host portion, and thus is the broadcast address for this
network. Note that there are exactly 14 usable addresses to assign to hosts.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
14
Determining the ‘Range’ of Subnetted Networks (continued)
Calculating the ranges of subnetted networks can quickly become tedious
when using the ‘long’ binary method. The ‘shortcut’ method involves taking
the subnet mask (255.255.255.240 from the previous example), and
subtracting the subnetted octet (240) from 256.
256 – 240 = 16
Assuming ip subnet-zero is enabled, the first network will begin at 0. Then,
simply continue adding 16 to list the first address of each new network:
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
Knowing the first address of each new network makes it simple to determine
the last address of each network:
First address of network 0 16 32 48 64 80 96 112 128 144
Last address of network 15 31 47 63 79 95 111 127 143 159
Only the first 10 networks were calculated, for brevity. The first address of
each network becomes the subnet address for that network. The last address
of each network becomes the broadcast address for that network.
Once the first and last address of each network is known, determining the
usable range for hosts is straightforward:
Subnet address 0 16 32 48 64 80 96 112 128 144
Usable Range 1 14 17 30 33 46 49 62 65 78 81 94 97 110 113 126 129 142 145 158
Broadcast address 15 31 47 63 79 95 111 127 143 159
Hosts on the same network (such as 192.168.254.2 and 192.168.254.14) can
communicate freely.
Hosts on different networks (such as 192.168.254.61 and 192.168.254.66)
require a router to communicate.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
15
Class A Subnetting Example
Consider the following subnetted Class A network: 10.0.0.0 255.255.248.0
Now consider the following questions:
• How many new networks were created?
• How many usable hosts are there per network?
• What is the full range of the first three networks?
By default, the 10.0.0.0 network has a subnet mask of 255.0.0.0. To
determine the number of bits stolen:
255.0.0.0: 11111111.00000000.00000000.00000000 255.255.248.0: 11111111.11111111.11111000.00000000
Clearly, 13 bits have been stolen to create the new subnet mask. To calculate
the total number of new networks:
2n = 213
= 8192 new networks created
There are clearly 11 bits remaining in the host portion of the mask:
2n – 2 = 211 – 2 = 2048 – 2 = 2046 usable hosts per network
Calculating the ranges is a bit tricky. Using the ‘shortcut’ method, subtract
the third octet (248) of the subnet mask (255.255.248.0) from 256.
256 – 248 = 8
The first network will begin at 0, again. However, the ranges are spread
across multiple octets. The ranges of the first three networks look as follows:
Subnet address 10.0.0.0 10.0.8.0 10.0.16.0
Usable Range 10.0.0.1 10.0.7.254 10.0.8.1 10.0.15.254 10.0.16.1 10.0.23.254
Broadcast address 10.0.7.255 10.0.15.255 10.0.23.255
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
16
Private vs Public Addresses
The rapid growth of the Internet resulted in a shortage of IPv4 addresses. In
response, the powers that be designated a specific subset of the IPv4 address
space to be private, to temporarily alleviate this problem.
A public address can be routed on the Internet. Thus, devices that should be
Internet accessible (such as web or email servers) must be configured with
public addresses.
A private address is only intended for use within an organization, and can
never be routed on the internet. Three private addressing ranges were
allocated, one for each IPv4 class:
• Class A - 10.x.x.x
• Class B - 172.16-31.x.x
• Class C - 192.168.x.x
NAT (Network Address Translation) is used to translate between private
addresses and public addresses. NAT allows devices configured with a
private address to be stamped with a public address, thus allowing those
devices to communicate across the Internet. NAT is covered in-depth in
another guide.
NAT is only a temporarily solution to the address shortage problem.
Eventually, IPv4 will be replaced with IPv6. This also is covered extensively
in another guide.
Two other ranges, while not considered “private,” have been reserved for
specific use:
• 127.x.x.x - reserved for diagnostic purposes. One such address
(127.0.0.1), identifies the local host, and is referred to as the loopback
or localhost address.
• 169.254.x.x - reserved for Automatic Private IP Addressing (APIPA).
A host assigns itself an APIPA address if a DHCP server is
unavailable to dynamically assign an address.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
17 The IPv4 Header
The IPv4 header has 12 required fields and 1 optional field¸ and is 160 bits
long. Field Length Description
Version 4 bits Version of IP (in this case, IPv4)
Header Length 4 bits Specifies the length of the IP header (minimum 160 bits)
Type of Service 8 bits Classifies traffic for QoS
Total Length 16 bits Specifies the length of both the header and data payload
Identification 16 bits Uniquely identifies fragments of a packet
Flags 3 bits Flags for fragmentation
Fragment Offset 13 bits Identifies the location of a fragment in a packet
Time to Live 8 bits Decremented by each router traversed
Protocol 8 bits Specifies the next upper layer protocol
Header Checksum 16 bits Checksum for error checking
Source Address 32 bits Source IPv4 address
Destination Address 32 bits Destination IPv4 address
Options 32 bits Optional field for various parameters
The Identification, Flags, and Fragment Offset fields are used in
conjunction with each other. An IP packet larger than the MTU size of a link
must be fragmented. Each fragment of the packet is marked with the same
Identification number. The Fragment Offset allows the destination device to
reassemble the fragments in the proper order.
The Flags field can dictate two conditions:
• Don’t Fragment (DF) – indicates the packet cannot be fragmented. If
the packet reaches a link with a small MTU, the packet is then
dropped, and an ICMP error message is sent back to the source.
• More Fragments (MF) – all fragments have this bit set to one, except
for the last fragment, where the bit is set to zero. This allows the
destination device to know it has received all fragments.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
18 IPv4 Protocol Numbers
The Next Header field is of some importance. This field identifies the next
upper-layer header (for example, UDP, TCP or ICMP). These upper layer
protocols are identified using IP Protocol Numbers.
The following is a list of common IP Protocol Numbers:
Protocol Number Upper-Layer Protocol 1 ICMP 2 IGMP 6 TCP 9 IGRP 17 UDP 46 RSVP 47 GRE 50 IPSEC ESP 51 IPSEC AH 88 EIGRP 89 OSPF (Reference: http://www.iana.org/assignments/protocol-numbers)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
19
Resolving Logical Addresses to Hardware Addresses
Hosts cannot directly send data to another device’s logical address. Network
communication occurs across the data-link layer, using hardware addresses.
A mechanism is required to map logical addresses to hardware addresses.
When using IP over an Ethernet network, the Address Resolution Protocol
(ARP) provides this function for us. ARP allows a host to determine the
MAC (hardware) address for a particular IP (logical) address.
Observe the above diagram. Following the step-by-step path a packet travels
from HostA to the 10.2.1.5 address (HostB):
• First, HostA determines if the 10.2.1.5 address is itself. If the address
is configured on a local interface, the packet never leaves HostA.
• Second, HostA determines if the 10.2.1.5 address is on the same
network (or subnet). If it is, HostA will broadcast an ARP request, and
wait for the appropriate host to reply with its MAC address.
• HostA determines that the 10.2.1.5 address is indeed on a separate
network. It now parses its local routing table for a route to this remote
network. Usually, hosts will be equipped with a default route (or,
default gateway), to reach all other networks.
• Host A determines that RouterA is its default gateway. The host
broadcasts an ARP request for RouterA’s MAC address, and then
forwards the packet to RouterA’s MAC (4444.5555.6666).
• RouterA receives the packet, and parses at its own routing table. It
determines that the 10.2.x.x network is directly attached off of its fa1
interface. The router then broadcasts an ARP request for the 10.2.1.5
address.
• HostB responds to the router’s ARP request with its MAC address
(AAAA.BBBB.CCCC). RouterA is then able to forward the packet to
HostB.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
20
Troubleshooting IP using ICMP
Internet Control Message Protocol (ICMP) is used for a multitude of
informational and error messaging purposes.
The following is a list of common ICMP types and codes:
Type Code Description
0 0 Echo Reply
- Destination Unreachable 0 Network Unreachable 1 Host Unreachable 2 Protocol Unreachable 3 Port Unreachable
4 Fragmentation Needed – Don’t Fragment Flag Set
6 Destination Network Unknown
7 Destination Host Unknown
9 Destination Network Administratively Prohibited
10 Destination Host Administratively Prohibited
3 5 Redirect 8 Echo 11 TTL Exceeded
Several IP troubleshooting tools utilize ICMP, including Packet Internet
Groper (ping) and traceroute.
Ping utilizes the Echo Request and Echo Reply ICMP messages to
determine if a host is responding on a particular address.
Traceroute determines the routing path a packet takes to reach its
destination.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
21 Section 2
- IPv6 Addressing -
IPv6 Basics
The most widespread implementation of IP currently is IPv4, which utilizes
a 32-bit address. Mathematically, a 32-bit address can provide roughly 4
billion unique IP addresses (232 = 4,294,967,296). Practically, the number of
usable IPv4 addresses is much lower, as many addresses are reserved for
diagnostic, experimental, or multicast purposes.
The explosive growth of the Internet and corporate networks quickly led to
an IPv4 address shortage. Various solutions were developed to alleviate this
shortage, including CIDR, NAT, and Private Addressing. However, these
solutions could only serve as temporary fixes.
In response to the address shortage, IPv6 was developed. IPv6 increases the
address size to 128 bits, providing a nearly unlimited supply of addresses
(340,282,366,920,938,463,463,374,607,431,768,211,456 to be exact). This
provides roughly 50 octillion addresses per person alive on Earth today, or
roughly 3.7 x 1021 addresses per square inch of the Earth’s surface.
(References: http://cc.uoregon.edu/cnews/spring2001/whatsipv6.html; http://en.wikipedia.org/wiki/IPv6)
IPv6 offers the following features:
• Increased Address Space and Scalability – providing the absurd
number of possible addresses stated previously.
• Simplified Configuration – allows hosts to auto-configure their IPv6
addresses, based on network prefixes advertised by routers.
• Integrated Security – provides built-in authentication and encryption
into the IPv6 network header
• Compatibility with IPv4 – simplifies address migration, as IPv6 is
backward-compatible with IPv4
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
22 The IPv6 Address
The IPv6 address is 128 bits, as opposed to the 32-bit IPv4 address. Also
unlike IPv4, the IPv6 address is represented in hexadecimal notation,
separate by colons.
An example of an IPv6 address would be:
1254:1532:26B1:CC14:0123:1111:2222:3333
Each “grouping” (from here on called fields) of hexadecimal digits is 16
bits, with a total of eight fields. The hexadecimal values of an IPv6 address
are not case-sensitive.
We can drop any leading zeros in each field of an IPv6 address. For
example, consider the following address:
1423:0021:0C13:CC1E:3142:0001:2222:3333
We can condense that address to: 1423:21:C13:CC1E:3142:1:2222:3333
Only leading zeros can be condensed. If we have an entire field comprised of
zeros, we can further compact the following address:
F12F:0000:0000:CC1E:2412:1111:2222:3333
The condensed address would be: F12F::CC1E:2412:1111:2222:3333
Notice the double colons (::). We can only condense one set of contiguous
zero fields. Thus, if we had the following address:
F12F:0000:0000:CC1E:2412:0000:0000:3333
We could not condense that to: F12F::CC1E:2412::3333
The address would now be ambiguous, as we wouldn’t know how many “0”
fields were compacted in each spot. Remember that we can only use one set
of double colons in an IPv6 address!
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
23 The IPv6 Prefix
IPv4 utilizes a subnet mask to define the network “prefix” and “host”
portions of an address. This subnet mask can also be represented in Classless
Inter-Domain Routing (CIDR) format.
IPv6 always use CIDR notation to determine what bits notate the prefix of
an address: Full Address: 1254:1532:26B1:CC14:123:1111:2222:3333/64 Prefix ID: 1254:1532:26B1:CC14: Host ID: 123:1111:2222:3333
The /64 indicates that the first 64 bits of this address identify the prefix.
The IPv6 Interface ID and EUI-64 Format
The host portion of an IPv4 address is not based on the hardware address of
an interface. IPv4 relies on Address Resolution Protocol (ARP) to map
between the logical IP address and the 48-bit hardware MAC address.
IPv6 unicasts generally allocate the first 64 bits of the address to identify the
network (prefix), and the last 64 bits to identify the host (referred to as the
interface ID). The interface ID is based on the interface’s hardware address.
This interface ID adheres to the IEEE 64-bit Extended Unique Identifier
(EUI-64) format. Since most interfaces still use the 48-bit MAC address, the
MAC must be converted into the EUI-64 format.
Consider the following MAC address: 1111.2222.3333. The first 24 bits, the
Organizationally Unique Identifier (OUI), identify the manufacturer. The
last 24 bits uniquely identify the host. To convert this to EUI-64 format:
1. The first 24 bits of the MAC (the OUI), become the first 24 bits of
the EUI-64 formatted interface ID.
2. The seventh bit of the OUI is changed from a “0” to a “1”.
3. The next 16 bits of the interface ID are FFFE.
4. The last 24 bits of the MAC (the host ID), become the last 24 bits of
the interface ID.
Thus, the MAC address 1111.2222.3333 in EUI-64 format would become
1311:22FF:FE22:3333, which becomes the interface ID.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
24
The IPv6 Address Hierarchy
IPv4 separated its address space into specific classes. The class of an IPv4
address was identified by the high-order bits of the first octet:
• Class A - (00000001 – 01111111, or 1 - 127)
• Class B - (10000000 – 10111111, or 128 - 191)
• Class C - (11000000 – 11011111, or 192 - 223)
• Class D - (11100000 – 11101111, or 224 - 239)
IPv6’s addressing structure is far more scalable. Less than 20% of the IPv6
address space has been designated for use, currently. The potential for
growth is enormous.
The address space that has been allocated is organized into several types,
determined by the high-order bits of the first field:
• Special Addresses – addresses begin 00xx:
• Link Local – addresses begin FE8x:
• Site Local – addresses begin FECx:
• Aggregate Global – addresses begin 2xxx: or 3xxx:
• Multicasts – addresses begin FFxx:
• Anycasts
(Note: an “x” indicates the value can be any hexadecimal number)
There are no broadcast addresses in IPv6. Thus, any IPv6 address that is
not a multicast is a unicast address.
Anycast addresses identify a group of interfaces on multiple hosts. Thus,
multiple hosts are configured with an identical address. Packets sent to an
anycast address are sent to the nearest (i.e., least amount of hops) host.
Anycasts are indistinguishable from any other IPv6 unicast address.
Practical applications of anycast addressing are a bit murky. One possible
application would be a server farm providing an identical service or
function, in which case anycast addressing would allow clients to connect to
the nearest server.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
25
Special (Reserved) IPv6 Addresses
The first field of a reserved or special IPv6 address will always begin 00xx.
Reserved addresses represent 1/256th of the available IPv6 address space.
Various reserved addresses exist, including:
• 0:0:0:0:0:0:0:0 (or ::) – is an unspecified or unknown address. It is
the equivalent of the IPv4 0.0.0.0 address, which indicates the absence
of a configured or assigned address. In routing tables, the unspecified
address is used to identify all or any possible hosts or networks.
• 0:0:0:0:0:0:0:1 (or ::1) – is the loopback or localhost address. It is
the equivalent of the IPv4 127.0.0.1 address.
Reserved Addresses - IPv4 and IPv6 Compatibility
To alleviate the difficulties of immediately migrating from IPv4 to IPv6,
specific reserved addresses can be used to embed an IPv4 address into an
IPv6 address.
Two types of addresses can be used for IPv4 embedding, IPv4-compatible
IPv6 addresses, and IPv4-mapped IPv6 addresses.
• 0:0:0:0:0:0:a.b.c.d (or ::a.b.c.d) – is an IPv4-compatible IPv6
address. This address is used on devices that support both IPv4 and
IPv6. A prefix of /96 is used for IPv4-compatible IPv6 addresses:
::192.168.1.1/96
• 0:0:0:0:0:FFFF:a.b.c.d (or ::FFFF:a.b.c.d) – is an IPv6-mapped
IPv6 address. This address is used by IPv6 routers and devices to
identify non-IPv6 capable devices. Again, a prefix of /96 is used for
IPv4-mapped IPv6 addresses: ::FFFF:192.168.1.1/96
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
26 Link-Local IPv6 Addresses
Link-local IPv6 addresses are used only on a single link (subnet). Any
packet that contains a link-local source or destination address is never routed
to another link. Every IPv6-enabled interface on a host (or router) is
assigned a link-local address. This address can be manually assigned, or
auto-configured.
The first field of a link-local IPv6 address will always begin FE8x (1111
1110 10). Link-local addresses are unicasts, and represent 1/1024th of the
available IPv6 address space. A prefix of /10 is used for link-local addresses.
FE80::1311:22FF:FE22:3333/10
There is no hierarchy to a link-local address:
• The first 10 bits are fixed (FE8), known as the Format Prefix (FP).
• The next 54 bits are set to 0.
• The final 64 bits are used as the interface ID.
Site Local IPv6 Addresses
Site-local IPv6 addresses are the equivalent of “private” IPv4 addresses.
Site-local addresses can be routed within a site or organization, but cannot
be globally routed on the Internet. Multiple private subnets within a “site”
are allowed.
The first field of a site-local IPv6 address will always begin FECx (1111
1110 11). Site-local addresses are unicasts, and represent 1/1024th of the
available IPv6 address space.
FEC0::2731:E2FF:FE96:C283/64
Site-local addresses do adhere to a hierarchy:
• The first 10 bits are the fixed FP (FEC).
• The next 38 bits are set to 0.
• The next 16 bits are used to identify the private subnet ID.
• The final 64 bits are used as the interface ID.
To identify two separate subnets (1111 and 2222):
FEC0::1111:2731:E2FF:FE96:C283/64 FEC0::2222:97A4:E2FF:FE1C:E2D1/64
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
27
Aggregate Global IPv6 Addresses
Aggregate Global IPv6 addresses are the equivalent of “public” IPv4
addresses. Aggregate global addresses can be routed publicly on the Internet.
Any device or site that wishes to traverse the Internet must be uniquely
identified with an aggregate global address.
Currently, the first field of an aggregate global IPv6 address will always
begin 2xxx (001). Aggregate global addresses are unicasts, and represent
1/8th of the available IPv6 address space.
2000::2731:E2FF:FE96:C283/64
Aggregate global addresses adhere to a very strict hierarchy:
• The first 3 bits are the fixed FP.
• The next 13 bits are the top-level aggregation identifier (TLA ID).
• The next 8 bits are reserved for future use.
• The next 24 bits are the next-level aggregation identifier (NLA ID).
• The next 16 bits are the site-level aggregation identifier (SLA ID).
• The final 64 bits are used as the interface ID.
By have multiple levels, a consistent, organized, and scalable hierarchy is
maintained. High level registries are assigned ranges of TLA IDs. These can
then be subdivided in the NLA ID field, and passed on to lower-tiered ISPs.
Such ISPs allocate these prefixes to their customers, which can further
subdivide the prefix using the SLA ID field, to create whatever local
hierarchy they wish. The 16-bit SLA field provides up to 65535 networks for
an organization.
Note: Do not confuse the SLA ID field of a global address field, with a sitelocal
address. Site-local addresses cannot be routed publicly, where as SLA
ID’s are just a subset of the publicly routable aggregate global address.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
28 Multicast IPv6 Addresses
Multicast IPv6 addresses are the equivalent of IPv4 multicast addresses.
Interfaces can belong to one or more multicast groups. Interfaces will accept
a multicast packet only if they belong to that group. Multicasting provides a
much more efficient mechanism than broadcasting, which requires that
every host on a link accept and process each broadcast packet.
The first field of a multicast IPv6 address will always begin FFxx (1111
1111). The full multicast range is FF00 through FFFF. Multicasts represent
1/256th of the available IPv6 address space.
FF01:0:0:0:0:0:0:1
Multicast addresses follow a specific format:
• The first 8 bits identify the address as a multicast (1111 1111)
• The next 4 bits are a flag value. If the flag is set to all zeroes (0000),
the multicast address is considered well-known.
• The next 4 bits are a scope value:
o 0000 (0) = Reserved
o 0001 (1) = Node Local Scope
o 0010 (2) = Link Local Scope
o 0101 (5) = Site Local Scope
o 1000 (8) = Organization Local Scope
o 1110 (e) = Global Scope
o 1111 (f) = Reserved
• The final 112 bits identify the actual multicast group.
IPv4 multicast addresses had no mechanism to support multiple “scopes.”
IPv6 scopes allow for a multicast hierarchy, a way to contain multicast
traffic.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
29
Common IPv6 Multicast Addresses
The following is a list of common, well-known IPv6 multicast addresses:
Node-Local Scope Multicast Addresses
• FF01::1 – All-nodes address
• FF01::2 – All-routers address
Link-Local Scope Multicast Addresses
• FF02::1 – All-nodes address
• FF02::2 – All-routers address
• FF02::5 – OSPFv3 (OSPF IPv6) All SPF Routers
• FF02::6 – OSPFv3 Designated Routers
• FF02::9 – RIPng Routers
• FF02::13 – PIM Routers
Site-Local Scope Multicast Addresses
• FF05::2 – All-routers address
All hosts must join the all-nodes multicast group, for both the node-local
and link-local scopes. All routers must join the all-routers multicast group,
for the node-local, link-local, and site-local scopes.
Every site-local and aggregate global address is assigned a solicited-node
multicast address. This solicited-node address is created by appending the
last 24 bits of the interface ID to the following prefix: FF02::1:FF/103.
Thus, if you have a site-local address of:
FEC0::1111:2731:E2FF:FE96:C283
The corresponding solicited-node multicast address would be:
FF02::1:FF96:C283
Solicited-node multicast addresses are most often used for neighbor
discovery (covered in an upcoming section in this guide).
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
30 Required IPv6 Addresses
At a minimum, each IPv6 interface on a host must recognize the following
IPv6 addresses:
• The loopback address
• A link-local address
• Any configured site-local or aggregate global addresses
• Any configured multicast groups
• The all-nodes multicast address (both node-local and link-local
scopes)
• The solicited-node multicast address for any configured unicast
addresses
In addition to the above addresses, each IPv6 interface on a router must
recognize the following IPv6 addresses:
• The subnet-router anycast address
• Any configured multicast groups
• The all-routers multicast address (node-local, link-local, and site-local
scopes)
IPv6 Addresses and URLs
IPv6 addresses can also be referenced in URLs (Uniform Resource Locator).
URL’s, however, use the colon to represent a specific TCP “port”. This is
not an issue with IPv4 addresses, which can easily be referenced using a
URL: http://192.168.1.1/index.html
Because IPv6 fields are separated by colons, the IPv6 address must be
placed in brackets, to conform to the URL standard:
http://[FEC0::CC1E:2412:1111:2222:3333]/index.html
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
31 The IPv6 Header
The IPv6 header has 8 fields and is 320 bits long. It has been considerably
streamlined compared to its IPv4 counterpart, which has 12 fields and is 160
bits long. Field Length Description
Version 4 bits Version of IP (in this case, IPv6)
Traffic Class 8 bits Classifies traffic for QoS
Flow Label 20 bits Identifies a flow between a source and destination
Payload Length 16 bits Length of data in packet
Next Header 8 bits Specifies the next upper-layer or extension header
Hop Limit 8 bits Decremented by each router traversed
Source Address 128 bits Source IPv6 address
Destination Address 128 bits Destination IPv6 address
The Next Header field is of some importance. This field can identify either
the next upper-layer header (for example, UDP, TCP or ICMP), or it can
identify a special Extension Header, which placed in between the IPv6 and
upper layer header.
Several such extension headers exist, and are usually processed in the
following order:
• Hop-by-Hop Options – specifies options that should be processed by
every router in the path. Directly follows the IPv6 header.
• Destination Options – specifies options that should be processed by
the destination device.
• Routing Header – specifies each router the packet must traverse to
reach the destination (source routing)
• Fragment Header – used when a packet is larger than the MTU for
the path
• Authentication Header – used to integrate IPSEC Authentication
Header (AH) into the IPv6 packet
• ESP Header – used to integrate IPSEC Encapsulating Security
Payload (ESP) into the IPv6 packet
(Reference: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t2/ipv6/ftipv6o.htm#1004285)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
32 ICMPv6
ICMP Version 6 (ICMPv6) is a core component of IPv6. All devices
employing IPv6 must also integrate ICMPv6.
ICMPv6 provides many services, including (but not limited to):
• Error Messages
• Informational messages (such as echo replies for IPv6 ping)
• MTU Path Discovery
• Neighbor Discovery
There are four key ICMPv6 error messages:
• Destination Unreachable (ICMP packet type 1) – indicates that the
packet cannot be forwarded to its destination. The node sending this
message includes an explanatory code:
o 0 - No route to destination
o 1 - Access is administratively prohibited
o 3 - Address unreachable
o 4 - Port unreachable
• Packet Too Big (ICMP packet type 2) – indicates the packet is larger
than the MTU of the link. IPv6 routers do not fragment packets.
Instead, the Packet Too Big message is sent to the source (sending)
device, which then reduces (or fragments) the size of the packet to the
reported MTU. This message is used for Path MTU Discovery
(PMTUD).
• Time Exceeded (ICMP packet type 3) – indicates that the hop count
limit has been reached, usually indicating a routing loop
• Parameter Problem (ICMP packet type 4) – indicates an error in the
IPv6 header, or an IPv6 extension header. The node sending this
message includes an explanatory code:
o 0 - Erroneous header field
o 1 - Unrecognized next-header type
o 2 - Unrecognized IPv6 option
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080113b1c.shtml)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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33
Neighbor Discovery Protocol (NDP) and ICMPv6
The neighbor discovery protocol (NDP) provides a multitude of services
for IPv6 enabled devices, including:
• Automatic address configuration, and prefix discovery
• Duplicate address detection
• MTU discovery • Router discovery • Address resolution
NDP replaces many IPv4 specific protocols, such as DHCP and ARP. NDP
utilizes ICMPv6 to provide the above services.
Periodically, IPv6 routers send out Router Advertisements (RA’s) to both
announce their presence on a link, and to provide auto-configuration
information for hosts. This RA (ICMP packet type 134) is sourced from the
link-local address of the sending router, and sent to the link-scope all-nodes
multicast group. The sending router sets a hop limit of 255 on a RA;
however, the RA packet must not be forwarded outside the local link.
Hosts use RA’s to configure themselves, and add the router to its local
default router list. A host can request an RA by sending out a Router
Solicitation (RS, ICMP packet type 133) to the link-local all-routers
multicast address. A RS is usually sent when a host is not currently
configured with an IP address.
The RA messages contain the following information for hosts:
• The router’s link-layer address (to be added to the host’s default
router list)
• One or more network prefixes
• A lifetime (measured in seconds) for the prefix(es)
• The link MTU
Routers send Redirect messages to hosts, indicating a better route to a
destination. Hosts can have multiple routers in its default router list, but one
is chosen as the true default router. If this default router deems that another
router has a better route to the destination, it forwards the Redirect message
to the sending host.
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34
Neighbor Discovery Protocol (NDP) and ICMPv6 (continued)
Neighbor Solicitations (NS’s, ICMP packet type 135) are sent by hosts to
identify the link-layer address of a neighbor, and ensure its reachability. A
NS message’s source address is the link-local address of the sending host,
and the destination is the solicited-node multicast address of the destination
host.
A neighbor will reply to a NS with a Neighbor Advertisement (NA, ICMP
packet type 136). This process replaces the Address Resolution Protocol
(ARP) used by IPv4, and provides a far more efficient means to learn
neighbor address information.
Hosts additionally use the NS messages to detect duplicate addresses.
Before a host assigns itself an IPv6 address, it sends out a NS to ensure no
other host is configured with that address.
Autoconfiguration of Hosts
Hosts can be assigned IPv6 addresses one of two ways: manually, or using
autoconfiguration. Hosts learn how to autoconfigure themselves from
Router Advertisements (RA’s).
Two types of autoconfiguration exist, stateless and stateful.
When using Stateless Autoconfiguration, a host first assigns itself a linklocal
IPv6 address. It accomplishes this by combining the link-local prefix
(FE8) with its interface ID (MAC address in EUI-64 format).
The host then sends a Router Solicitation multicast to the all-routers
multicast address, which provides one or more network prefixes. The host
combines these prefixes with its interface ID to create its site-local (or
aggregate global) IPv6 addresses.
Stateful Autoconfiguration is used in conjunction with stateless
autoconfiguration. Stateful Autoconfiguration utilizes DHCPv6 to provide
additional information to the host, such as DNS servers. DHCPv6 can also
be used in the event that there is no router on the link, to provide stateless
autoconfiguration.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
35 Configuring IPv6 Addresses
IPv6 support is disabled by default on Cisco routers, and must be enabled
globally: Router(config)# ipv6 unicast-routing
To configure an interface to auto-configure a link-local IPv6 address:
Router(config)# interface e0 Router(config-if)# ipv6 enable
To manually configure a site-local IPv6 address on an interface:
Router(config)# interface e0
Router(config-if)# ipv6 address FEC0::/64 eui-64
The eui-64 parameter will append interface ID (MAC address in EUI-64
format) to the site-local prefix. Otherwise, we could have specified the full
IPv6 address:
Router(config-if)# ipv6 address FEC0::1:1234:23FF:FE21:1212 eui-64
Recall that we can configure multiple subnets for our site-local address
space: Router(config)# interface e0
Router(config-if)# ipv6 address FEC0::2222:0:0:0:0/64 eui-64
To configure a router interface to advertise a specific prefix to hosts on the
link: Router(config)# interface e0
Router(config-if)# ipv6 nd prefix-advertisement 2002:1111::/48 2000 1000 onlink autoconfig
The router will advertise a prefix of 2002:1111::/48 with a valid lifetime of
2000 seconds and a preferred lifetime of 1000 seconds. The clients will
autoconfig themselves based on this prefix.
To view IPv6 specific information about an interface:
Router# show ipv6 interface e0
To create a static host entry for an IPv6 address:
Router(config)# ipv6 host MYHOST FEC0::1111:2731:E2FF:FE96:C283
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
36
Configuring IPv6 Static Routes
The syntax to configure an IPv6 static route is simple:
Router(config)# ipv6 route FEC0::2222/64 FEC0::1111:3E5F:2E5B:A3D1
The above command creates an ipv6 route to the FEC0::2222/64 network,
with a next-hop of FEC0::1111:3E5F:2E5B:A3D1.
To create an IPv6 default route:
Router(config)# ipv6 route ::/0 FE80::2
The above command creates an ipv6 default route, with a next hop of
FE80::2. The ::/0 designation indicates all zeros in the address field, and a
mask of zero bits (the unspecified address).
To view the IPv6 routing table:
Router(config)# show ipv6 route
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
37 Configuring IPv6 RIPng
A version of RIP for IPv6 was developed called RIPng (RIP Next
Generation). Functionally, RIPng is the equivalent of RIPv2, with the
additional support for IPv6 addresses. However, RIPng is not backwards
with earlier version of RIP, and does not support IPv4 addressing.
Basic RIPng characteristics:
• Administrative distance of 120
• Maximum hopcount of 16
• Updates are sent every 30 seconds as multicasts
To configure RIPng, we must first enable the RIP process globally:
Router(config)# ipv6 router rip MYPROCESS
We are enabling an ipv6 rip process called MYPROCESS. Next, we must
enable RIPng on each participating interface:
Router(config)# interface e0
Router(config-if)# ipv6 rip MYPROCESS enable
RIPng, by default, utilizes UDP port 521 and multicast group FF02::9, but
these parameters can be changed globally:
Router(config)# ipv6 rip MYPROCESS port 555 multicast-group FF02::1111
We can adjust RIPng’s timers:
Router(config)# ipv6 rip MYPROCESS timers 30 180 180 120
In order, the above timers are update, expire, holddown, and garbagecollect.
The above values are default.
To control inbound or outbound RIPng updates, using an access-list:
Router(config)# interface e0
Router(config-if)# ipv6 rip MYPROCESS input-filter MYACCESSLIST
Router(config-if)# ipv6 rip MYPROCESS output-filter MYACCESSLIST
To view configuration and status information for RIPng:
Router# show ipv6 protocols
Router# show ipv6 rip
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
38
Configuring IPv6 OSPF (OSPFv3)
OSPFv2 is a widely used link-state routing protocol in IPv4 environments.
To support IPv6, OSPFv3 was developed. Its function is very similar to
OSPFv2.
First, we must first enable the OSPF process globally:
Router(config)# ipv6 router ospf 1
The 1 indicates the process ID. Next, we must place the participating
interfaces in their appropriate areas:
Router(config)# interface e0
Router(config-if)# ipv6 ospf 1 area 0
Router(config)# interface s0
Router(config-if)# ipv6 ospf 1 area 1
Please note: the Router ID for OSPFv3 is still a 32-bit value. Thus, the
highest IPv4 loopback address will be chosen first, then the highest IPv4
physical address. If neither exist, a 32-bit Router ID must be manually
specified:
Router(config)# ipv6 router ospf 1
Router(config-router)# router-id 1.1.1.1
To create a summarized route on an area boundary:
Router(config)# ipv6 router ospf 1
Router(config-router)# area range 2001:1111::/48
To view configuration and status information for OSPFv3:
Router# show ipv6 ospf neighbor
Router# show ipv6 ospf interface
To clear an OSPFv3 process:
Router# clear ipv6 ospf 1
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
39 Configuring IPv6 BGP
BGP-4 does not natively support IPv6. Support for IPv6 and other protocols
(such as IPX) are included in the BGP Multi-protocol Extensions.
Basic BGP configuration using IPv6 is identical to that of IPv4:
Router(config)# router bgp 100
Router(config-router)# neighbor 2005:2222::1 remote-as 200
Notice the use of an aggregate global IPv6 address in the neighbor
statement.
Additional information is required - we must activate the neighbor. This
allows the neighbor to share IPv6 routes with the local router:
Router(config)# router bgp 100
Router(config-router)# address-family ipv6
Router(config-router-af)# neighbor 2005:2222::1 activate
To advertise an IPv6 prefix into BGP:
Router(config)# router bgp 100
Router(config-router)# address-family ipv6
Router(config-router-af)# network 2005:1111:: /24
To view configuration and status information for IPv6 BGP:
Router# show bgp ipv6
Router# show bgp ipv6 summary
(Reference: http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_configuration_guide_chapter09186a00801d65f7.html)
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40
Configuring an IPv6 Tunnel
We can configure an IPv6 “tunnel” across an IPv4 link. To accomplish this,
we create a virtual tunnel interface on both RouterA and RouterB.
RouterA(config)# ipv6 unicast-routing RouterA(config)# interface fa0
RouterA(config-if)# ipv6 address FEC0:0:0:1111::/64 eui-64
RouterA(config)# interface fa1
RouterA(config-if)# ip address 10.1.1.1 255.255.0.0
RouterA(config)# interface tunnel0
RouterA(config-if)# no ip address
RouterA(config-if)# ipv6 address FEC0:0:0:2222::1/124
RouterA(config-if)# tunnel source fa1
RouterA(config-if)# tunnel destination 10.1.1.2
RouterA(config-if)# tunnel mode ipv6ip
Configuration on Router B:
RouterB(config)# ipv6 unicast-routing RouterB(config)# interface fa0
RouterB(config-if)# ip address 10.1.1.2 255.255.0.0
RouterB(config)# interface fa1
RouterB(config-if)# ipv6 address FEC0:0:0:3333::/64 eui-64
RouterB(config)# interface tunnel0
RouterB(config-if)# no ip address
RouterB(config-if)# ipv6 address FEC0:0:0:2222::2/124
RouterB(config-if)# tunnel source fa1
RouterB(config-if)# tunnel destination 10.1.1.1
RouterB(config-if)# tunnel mode ipv6ip
We’ve applied an IPv6 address on the FEC0:0:0:2222::/124 network. IPv6
traffic can now route across the 10.1.x.x/16 IPv4 network. Any routing
protocol configuration for IPv6 should be completed on the tunnel
interfaces.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
41 IPv6 Access-Lists
Cisco IOS 12.0(23) or later supports IPv6 access-lists. The configuration is
similar to that of IPv4 named access-lists (All IPv6 access-lists are named;
there are no IPv6 numbered access-lists).
Router(config)# ipv6 access-list MYLIST
Router(config-access-list)# deny ipv6 any 2001:1111::/64
Router(config-access-list)# permit ipv6 any any
Router(config)# interface fa0/0
Router(config-if)# ipv6 traffic-filter MYLIST in
Notice the use of a /prefix, as opposed to a wildcard mask.
Also, notice the use of the ipv6 traffic-filter command to apply the ACL to
the interface, as opposed to ip access-group.
Hurray for consistency! (Reference: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t2/ipv6/ftipv6c.htm#1064881)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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42 Section 3
- TCP and UDP -
Transport Layer Protocols
The Transport layer of the OSI model (or, the Host-to-Host layer of the
DoD model) is concerned with the reliable transfer of data between devices.
It ensures (or in some cases, does not ensure) that a packet arrives at its
destination without corruption or data loss.
However, protocols at the transport layer do not actually send or route
packets. Network layer protocols, such as IP, route packets from one
network to another. In the TCP/IP protocol suite, TCP and UDP are
transport layer protocols.
Transmission Control Protocol (TCP)
The Transmission Control Protocol (TCP) is defined as a reliable,
connection-oriented transport protocol. Parameters must be agreed upon by
both parties before a connection is established.
TCP utilizes a three-way handshake to accomplish this. Control messages
are passed between two devices as the connection is set up:
• Host A sends a SYN (short for synchronize) message to Host B to
initiate a connection
• Host B responds with an ACK (short for acknowledgement) to Host
A’s SYN message, and sends its own SYN message (both messages
are combined to form a SYN+ACK)
• Host A completes the three-way handshake by sending an ACK.
The TCP header contains both a SYN flag and an ACK flag. Thus, when a
particular message needs to be sent, the appropriate flag is marked as on (in
other words, changed from a “0” to a “1”). A SYN+ACK message has both
flags set to on (1).
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43
Transmission Control Protocol (TCP) (continued)
Additionally, TCP segments data into smaller pieces for transport. Segments
are assigned a sequence number, so that the receiving device can then
reassemble this data in order upon arrival.
• Host A sends an initial sequence number (ISN) with its SYN
message. This number is chosen from a random timer – we’ll assume
an ISN of 4000.
• Host B responds to this sequence number with an acknowledgment
number, which is always one more than the sequence number. Thus,
Host B’s acknowledgment number is 4001.
• Additionally, Host B sends an initial sequence number with its SYN
message. We’ll assume Host B’s ISN is 6000.
• Host A responds to this sequence with an acknowledgement number
of 6001.
After a TCP connection is established, each segment is tagged with a
sequence number. TCP detects that a segment has been lost when it does not
receive a corresponding acknowledgement of receipt. It must not only
receive an ACK, but it must receive an ACK with the appropriate
acknowledgement number. (Reference: http://www.tcpipguide.com/free/t_TCPConnectionEstablishmentSequenceNumberSynchroniz.htm)
Additionally, TCP incorporates windowing for flow control. When flow
control is enabled, both the sending and receiving devices must agree on the
amount of data being sent in between acknowledgements. This helps prevent
data loss due to one side of the connection being overloaded.
(Reference: http://www.tcpipguide.com/free/t_TCPSlidingWindowAcknowledgmentSystemForDataTranspo.htm)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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44 The TCP Header
The TCP header has 12 fields:
Field Length Description
Source Port 16 bits Source TCP Port
Destination Port 16 bits Destination TCP Port
Sequence Number 32 bits Initial Sequence Number
Ack Number 32 bits Acknowledgement Number
Data Offset 4 bits Indicates where the data begins in a TCP segment
Reserved 6 bits Always set to 0
Control Bits 6 bits URG, ACK, PSH, RST, SYN, and FIN flags
Window 16 bits Used for Flow Control
Checksum 16 bits Used for Error-Checking
Urgent Pointer 16 bits
Options Variable
Padding Variable To ensure the TCP header ends at a 32 bit boundary
User Datagram Protocol (UDP)
The User Datagram Protocol (UDP) is defined as an unreliable,
connectionless transport protocol. It is essentially a stripped-down version
of TCP, and thus has far less latency than TCP.
UDP provides no three-way handshake, no flow-control, no sequencing, and
no acknowledgment of data receipt. However, UDP does provide basic
error-checking using a checksum.
The UDP header has only 4 fields:
Field Length Description
Source Port 16 bits Source UDP Port
Destination Port 16 bits Destination UDP Port
Length 16 bits Length of the header and the data
Checksum 16 bits Used for Error-Checking
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45
Comparison of TCP versus UDP
TCP UDP Connection-oriented Connection-less
Guaranteed Delivery No Guaranteed Delivery
Sends Acknowledgments Does not send Acknowledgments
Reliable, but slow Unreliable, but fast
Segments and Sequences Data Does NOT segment/sequence data
Flow Control No Flow Control
Performs CRC on data Performs CRC on data
TCP/UDP Ports
TCP and UDP ports identify services that run on a specific logical address.
Otherwise, there would be no way to distinguish data destined for one
service or another on a device. For example, port numbers allow both a web
and email server to operate simultaneously on the same address.
An IP address combined with a TCP or UDP port forms a socket. A socket
is written out as follows:
10.50.1.1:80
Specific ports (1-1024) have been reserved for specific services, and are
recognized as well-known ports. Below is a table of several common
TCP/UDP ports:
20, 21 TCP FTP
22 TCP SSH 23 TCP Telnet 25 TCP SMTP 53 UDP DNS 80 TCP HTTP 110 TCP POP3 443 TCP SSL 666 TCP Doom
For a complete list of port numbers, refer to the IANA website:
http://www.iana.org/assignments/port-numbers.
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46 ________________________________________________ Part II Basic Routing Concepts ________________________________________________
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
47 Section 4
- The Routing Table -
Routing Table Basics
Routing is the process of sending a packet of information from one network
to another network. Thus, routes are usually based on the destination
network, and not the destination host (host routes can exist, but are used
only in rare circumstances).
To route, routers build Routing Tables that contain the following:
• The destination network and subnet mask
• The “next hop” router to get to the destination network
• Routing metrics and Administrative Distance
The routing table is concerned with two types of protocols:
• A routed protocol is a layer 3 protocol that applies logical addresses
to devices and routes data between networks. Examples would be IP
and IPX.
• A routing protocol dynamically builds the network, topology, and
next hop information in routing tables. Examples would be RIP,
IGRP, OSPF, etc.
To determine the best route to a destination, a router considers three
elements (in this order):
• Prefix-Length
• Metric (within a routing protocol)
• Administrative Distance (between separate routing protocols)
Prefix-length is the number of bits used to identify the network, and is used
to determine the most specific route. A longer prefix-length indicates a more
specific route. For example, assume we are trying to reach a host address of
10.1.5.2/24. If we had routes to the following networks in the routing table:
10.1.5.0/24 10.0.0.0/8
The router will do a bit-by-bit comparison to find the most specific route
(i.e., longest matching prefix). Since the 10.1.5.0/24 network is more
specific, that route will be used, regardless of metric or Administrative
Distance.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
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48
Administrative Distance vs. Metric
A “metric” allows a router to choose the best path within a routing protocol.
Distance vector routing protocols use “distance” (usually hop-count) as their
metric. Link state protocols utilize some sort of “cost” as their metric.
Only routes with the best metric are added to the routing table. Thus, even
if a particular routing protocol (for example, RIP) has four routes to the
same network, only the route with the best metric (hop-count in this
example) would make it to the routing table. If multiple equal-metric routes
exist to a particular network, most routing protocols will load-balance.
If your router is running multiple routing protocols, Administrative
Distance is used to determine which routing protocol to trust the most.
Lowest administrative distance wins.
Again: if a router receives two RIP routes to the same network, it will use
the routes’ metric to determine which path to use. If the metric is identical
for both routes, the router will load balance between both paths.
If a router receives a RIP and an OSPF route to the same network, it will use
Administrative Distance to determine which routing path to choose.
The Administrative Distance of common routing protocols (remember,
lowest wins): Connected 0 Static 1 EIGRP Summary 5 External BGP 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 External EIGRP 170 Internal BGP 200 Unknown 255
A route with an “unknown” Administrative Distance will never be inserted
into the routing table.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
49
Viewing the routing table
The following command will allow you to view the routing table:
Router# show ip route
Gateway of last resort is 192.168.1.1 to network 0.0.0.0
C 192.168.1.0/24 is directly connected, Ethernet0
150.50.0.0/24 is subnetted, 1 subnets
C 150.50.200.0 is directly connected, Loopback1
C 192.168.123.0 is directly connected, Serial0
C 192.168.111.0 is directly connected, Serial1
R 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0
[120/1] via 192.168.111.2, 00:00:00, Serial1
S* 0.0.0.0/0 [1/0] via 192.168.1.1
Routes are labeled based on what protocol placed them in the table:
o C – Directly connected
o S – Static
o S* - Default route
o D - EIGRP
o R – RIP
o I – IGRP
o i – IS-IS
o O - OSPF
Notice the RIP routes contain the following field: [120/1]. This indicates
both the administrative distance and the metric (the 120 is the AD, and the 1
is the hop-count metric).
To clear all routes from the routing table, and thus forcing any routing
protocol to repopulate the table:
Router# clear ip route *
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
50
Choosing the Best Route (Example)
Assume the following routes existed to the following host: 192.168.111.5/24
O 192.168.111.0/24 [110/58] via 192.168.131.1, 00:00:00, Serial3
R 192.168.111.0/24 [120/1] via 192.168.123.1, 00:00:00, Serial0
R 192.168.111.0/24 [120/5] via 192.168.5.2, 00:00:00, Serial1
S 192.168.0.0/16 [1/0] via 10.1.1.1
We have two RIP routes, an OSPF route, and a Static route to that
destination. Which route will be chosen by the router?
Remember the three criteria the router considers:
• Prefix-Length • Metric • Administrative Distance
The static route has the lowest administrative distance (1) of any of the
routes; however, its prefix-length is less specific. 192.168.111.0/24 is a
more specific route than 192.168.0.0/16. Remember, prefix-length is always
considered first.
The second RIP route will not be inserted into the routing table, because it
has a higher metric (5) than the first RIP route (1). Thus, our routing table
will actually look as follows:
O 192.168.111.0/24 [110/58] via 192.168.131.1, 00:00:00, Serial3
R 192.168.111.0/24 [120/1] via 192.168.123.1, 00:00:00, Serial0
S 192.168.0.0/16 [1/0] via 10.1.1.1
Thus, the true choice is between the OSPF route and the first RIP route.
OSPF has the lowest administrative distance, and thus that route will be
preferred.
PLEASE NOTE: Calculating the lowest metric route within a routing
protocol occurs before administrative distance chooses the route it “trusts”
the most. This is why the order of the above “criteria” is prefix-length,
metric, and then administrative distance.
However, the route with the lowest administrative distance is always
preferred, regardless of metric (assuming the prefix-length is equal). Thus,
the metric is calculated first, but not preferred first over AD.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
51 Section 5
- Classful vs. Classless Routing -
Classful vs Classless routing protocols
Classful routing protocols do not send subnet mask information with their
routing updates. A router running a classful routing protocol will react in one
of two ways when receiving a route:
• If the router has a directly connected interface belonging to the same
major network, it will apply the same subnet mask as that interface.
• If the router does not have any interfaces belonging to the same major
network, it will apply the classful subnet mask to the route.
Belonging to same “major network” simply indicates that they belong to the
same “classful” network. For example:
• 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0)
• 10.1.4.5 and 11.1.4.4 do not belong to the same major network
• 192.168.1.1 and 192.168.1.254 belong to the same major network
(192.168.1.0)
• 192.168.1.5 and 192.167.2.5 do not belong to the same major network
Take the following example (assume the routing protocol is classful):
If Router B sends a routing update to Router A, it will not include the subnet
mask for the 10.2.0.0 network. Thus, Router A must make a decision.
If Router A has a directly connected interface that belongs to the same major
network (10.0.0.0), it will use the subnet mask of that interface for the route.
For example, if Router A has an interface on the 10.4.0.0/16 network, it will
apply a subnet mask of /16 to the 10.2.0.0 network.
If Router A does not have a directly connected interfacing belonging to the
same major network, it will apply the classful subnet mask of /8. This can
obviously cause routing difficulties.
When using classful routing protocols, the subnet mask must remain
consistent throughout your entire network.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
52
Classful vs Classless routing protocols (continued)
Classless routing protocols do send the subnet mask with their updates.
Thus, Variable Length Subnet Masks (VLSMs) are allowed when using
classless routing protocols.
Examples of classful routing protocols include RIPv1 and IGRP.
Examples of classless routing protocols include RIPv2, EIGRP, OSPF, and
IS-IS.
The IP Classless Command
The preceding section described how classful and classless protocols differ
when sending routing updates. Additionally, the router itself can operate
either “classfully” or “classlessly” when actually routing data.
When a “classful” router has an interface connected to a major network, it
believes it knows all routes connected to that major network.
For example, a router may have an interface attached to the 10.1.5.0/24
network. It may also have routes from a routing protocol, also for the
10.x.x.x network.
However, if the classful router receives a packet destined for a 10.x.x.x
subnet that is not in the routing table, it will drop that packet, even if there is
a default route.
Again, a classful router believes it knows all possible destinations in a major
network.
To configure your router in “classful” mode:
Router(config)# no ip classless
To configure your router in “classless” mode (this is default in IOS 12.0 and
greater): Router(config)# ip classless (Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml)
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
53
Limitations of Classful Routing Example
The following section will illustrate the limitations of classful routing, using
RIPv1 as an example. Consider the following diagram:
This particular scenario will work when using RIPv1, despite the fact that
we’ve subnetted the major 10.0.0.0 network. Notice that the subnets are
contiguous (that is, they belong to the same major network), and use the
same subnet mask.
When Router A sends a RIPv1 update to Router B via Serial0, it will not
include the subnet mask for the 10.1.0.0 network. However, because the
10.3.0.0 network is in the same major network as the 10.1.0.0 network, it
will not summarize the address. The route entry in the update will simply
state “10.1.0.0”.
Router B will accept this routing update, and realize that the interface
receiving the update (Serial0) belongs to the same major network as the
route entry of 10.1.0.0. It will then apply the subnet mask of its Serial0
interface to this route entry.
Router C will similarly send an entry for the 10.2.0.0 network to Router B.
Router B’s routing table will thus look like:
RouterB# show ip route
Gateway of last resort is not set
10.0.0.0/16 is subnetted, 4 subnets
C 10.3.0.0 is directly connected, Serial0
C 10.4.0.0 is directly connected, Serial1
R 10.1.0.0 [120/1] via 10.3.5.1, 00:00:00, Serial0
R 10.2.0.0 [120/1] via 10.4.5.1, 00:00:00, Serial1
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
54
Limitations of Classful Routing Example
Consider the following, slightly altered, example:
We’ll assume that RIPv1 is configured correctly on all routers. Notice that
our networks are no longer contiguous. Both Router A and Router C contain
subnets of the 10.0.0.0 major network (10.1.0.0 and 10.2.0.0 respectively).
Separating these networks now are two Class C subnets (192.168.123.0 and
192.168.111.0).
Why is this a problem? Again, when Router A sends a RIPv1 update to
Router B via Serial, it will not include the subnet mask for the 10.1.0.0
network. Instead, Router A will consider itself a border router, as the
10.1.0.0 and 192.168.123.0 networks do not belong to the same major
network. Router A will summarize the 10.1.0.0/16 network to its classful
boundary of 10.0.0.0/8.
Router B will accept this routing update, and realize that it does not have a
directly connected interface in the 10.x.x.x scheme. Thus, it has no subnet
mask to apply to this route. Because of this, Router B will install the
summarized 10.0.0.0 route into its routing table.
Router C, similarly, will consider itself a border router between networks
10.2.0.0 and 192.168.111.0. Thus, Router C will also send a summarized
10.0.0.0 route to Router B.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
55
Limitations of Classful Routing Example
Router B’s routing table will then look like:
RouterB# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
C 192.168.111.0 is directly connected, Serial1
R 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0
[120/1] via 192.168.111.2, 00:00:00, Serial1
That’s right, Router B now has two equal metric routes to get to the
summarized 10.0.0.0 network, one through Router A and the other through
Router C. Router B will now load balance all traffic to any 10.x.x.x network
between routers A and C. Suffice to say, this is not a good thing.
It gets better. Router B then tries to send routing updates to Router A and
Router C, including the summary route of 10.0.0.0/8. Router A’s routing
table looks like:
RouterA# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
10.0.0.0/16 is subnetted, 1 subnet
C 10.1.0.0 is directly connected, Ethernet0
Router A will receive the summarized 10.0.0.0/8 route from Router B, and
will reject it. This is because it already has the summary network of 10.0.0.0
in its routing table, and it’s directly connected. Router C will respond
exactly the same, and the 10.1.0.0/16 and 10.2.0.0/16 networks will never be
able to communicate.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
56 Section 6
- Static vs. Dynamic Routing -
Static vs. Dynamic Routing
There are two basic methods of building a routing table:
• Static Routing • Dynamic Routing
A static routing table is created, maintained, and updated by a network
administrator, manually. A static route to every network must be configured
on every router for full connectivity. This provides a granular level of
control over routing, but quickly becomes impractical on large networks.
Routers will not share static routes with each other, thus reducing
CPU/RAM overhead and saving bandwidth. However, static routing is not
fault-tolerant, as any change to the routing infrastructure (such as a link
going down, or a new network added) requires manual intervention. Routers
operating in a purely static environment cannot seamlessly choose a better
route if a link becomes unavailable.
Static routes have an Administrative Distance (AD) of 1, and thus are always
preferred over dynamic routes, unless the default AD is changed. A static
route with an adjusted AD is called a floating static route.
A dynamic routing table is created, maintained, and updated by a routing
protocol running on the router. Examples of routing protocols include RIP
(Routing Information Protocol), EIGRP (Enhanced Interior Gateway
Routing Protocol), and OSPF (Open Shortest Path First). Specific dynamic
routing protocols are covered in great detail in other guides.
Routers do share dynamic routing information with each other, which
increases CPU, RAM, and bandwidth usage. However, routing protocols are
capable of dynamically choosing a different (or better) path when there is a
change to the routing infrastructure.
Do not confuse routing protocols with routed protocols:
• A routed protocol is a Layer 3 protocol that applies logical
addresses to devices and routes data between networks (such as IP)
• A routing protocol dynamically builds the network, topology, and
next hop information in routing tables (such as RIP, EIGRP, etc.)
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
57
Static vs. Dynamic Routing (continued)
The following briefly outlines the advantages and disadvantages of static
routing: Advantages of Static Routing
• Minimal CPU/Memory overhead
• No bandwidth overhead (updates are not shared
between routers)
• Granular control on how traffic is routed
Disadvantages of Static Routing
• Infrastructure changes must be manually adjusted
• No “dynamic” fault tolerance if a link goes down
• Impractical on large network
The following briefly outlines the advantages and disadvantages of dynamic
routing: Advantages of Dynamic Routing
• Simpler to configure on larger networks
• Will dynamically choose a different (or better)
route if a link goes down
• Ability to load balance between multiple links
Disadvantages of Dynamic Routing
• Updates are shared between routers, thus
consuming bandwidth
• Routing protocols put additional load on router
CPU/RAM
• The choice of the “best route” is in the hands of
the routing protocol, and not the network
administrator
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
58 Dynamic Routing Categories
There are two distinct categories of dynamic routing protocols:
• Distance-vector protocols • Link-state protocols
Examples of distance-vector protocols include RIP and IGRP. Examples of
link-state protocols include OSPF and IS-IS.
EIGRP exhibits both distance-vector and link-state characteristics, and is
considered a hybrid protocol.
Distance-vector Routing Protocols
All distance-vector routing protocols share several key characteristics:
• Periodic updates of the full routing table are sent to routing
neighbors.
• Distance-vector protocols suffer from slow convergence, and are
highly susceptible to loops.
• Some form of distance is used to calculate a route’s metric.
• The Bellman-Ford algorithm is used to determine the shortest path.
A distance-vector routing protocol begins by advertising directly-connected
networks to its neighbors. These updates are sent regularly (RIP – every 30
seconds; IGRP – every 90 seconds).
Neighbors will add the routes from these updates to their own routing tables.
Each neighbor trusts this information completely, and will forward their full
routing table (connected and learned routes) to every other neighbor. Thus,
routers fully (and blindly) rely on neighbors for route information, a concept
known as routing by rumor.
There are several disadvantages to this behavior. Because routing
information is propagated from neighbor to neighbor via periodic updates,
distance-vector protocols suffer from slow convergence. This, in addition to
blind faith of neighbor updates, results in distance-vector protocols being
highly susceptible to routing loops.
Distance-vector protocols utilize some form of distance to calculate a
route’s metric. RIP uses hopcount as its distance metric, and IGRP uses a
composite of bandwidth and delay.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
59 Link-State Routing Protocols
Link-state routing protocols were developed to alleviate the convergence
and loop issues of distance-vector protocols. Link-state protocols maintain
three separate tables:
• Neighbor table – contains a list of all neighbors, and the interface
each neighbor is connected off of. Neighbors are formed by sending
Hello packets.
• Topology table – otherwise known as the “link-state” table, contains
a map of all links within an area, including each link’s status.
• Shortest-Path table – contains the best routes to each particular
destination (otherwise known as the “routing” table”)
Link-state protocols do not “route by rumor.” Instead, routers send updates
advertising the state of their links (a link is a directly-connected network).
All routers know the state of all existing links within their area, and store
this information in a topology table. All routers within an area have identical
topology tables.
The best route to each link (network) is stored in the routing (or shortestpath)
table. If the state of a link changes, such as a router interface failing,
an advertisement containing only this link-state change will be sent to all
routers within that area. Each router will adjust its topology table
accordingly, and will calculate a new best route if required.
By maintaining a consistent topology table among all routers within an area,
link-state protocols can converge very quickly and are immune to routing
loops.
Additionally, because updates are sent only during a link-state change, and
contain only the change (and not the full table), link-state protocols are less
bandwidth intensive than distance-vector protocols. However, the three
link-state tables utilize more RAM and CPU on the router itself.
Link-state protocols utilize some form of cost, usually based on bandwidth,
to calculate a route’s metric. The Dijkstra formula is used to determine the
shortest path.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
60 Section 7
- Configuring Static Routes -
Configuring Static Routes
The basic syntax for a static route is as follows:
Router(config)# ip route [destination_network] [subnet_mask] [next-hop]
Consider the following example:
RouterA will have the 172.16.0.0/16 and 172.17.0.0/16 networks in its
routing table as directly-connected routes. To add a static route on RouterA,
pointing to the 172.18.0.0/16 network off of RouterB:
RouterA(config)# ip route 172.18.0.0 255.255.0.0 172.17.1.2
Notice that we point to the IP address on RouterB’s fa0/0 interface as the
next-hop address. Likewise, to add a static route on RouterB, pointing to the
172.16.0.0/16 network off of RouterA:
RouterB(config)# ip route 172.16.0.0 255.255.0.0 172.17.1.1
To remove a static route, simply type no in front of it:
RouterA(config)# no ip route 172.18.0.0 255.255.0.0 172.17.1.2
On point-to-point links, an exit-interface can be specified instead of a nexthop
address. Still using the previous diagram as an example:
RouterA(config)# ip route 172.18.0.0 255.255.0.0 fa0/1
RouterB(config)# ip route 172.16.0.0 255.255.0.0 fa0/0
A static route using an exit-interface has an Administrative Distance of 0, as
opposed to the default AD of 1 for static routes. An exit-interface is only
functional on a point-to-point link, as there is only one possible next-hop
device.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
* * *
All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
61
Advanced Static Routes Parameters
The Administrative Distance of a static route can be changed to form a
floating static route, which will only be used if there are no other routes
with a lesser AD in the routing table. A floating static route is often used as a
backup route to a dynamic routing protocol.
To change the Administrative Distance of a static route to 250:
RouterA(config)# ip route 172.18.0.0 255.255.0.0 172.17.1.2 250
Static routes will only remain in the routing table as long as the interface
connecting to the next-hop router is up. To ensure a static route remains
permantly in the routing table, even if the next-hop interface is down:
RouterA(config)# ip route 172.18.0.0 255.255.0.0 172.17.1.2 permanent
Static routes can additionally be used to discard traffic to specific networks,
by directing that traffic to a virtual null interface:
RouterA(config)# ip route 10.0.0.0 255.0.0.0 null0
Default Routes
Normally, if a specific route to a particular network does not exist, a router
will drop all traffic destined to that network.
A default route, or gateway of last resort, allows traffic to be forwarded,
even without a specific route to a particular network.
The default route is identified by all zeros in both the network and subnet
mask (0.0.0.0 0.0.0.0). It is the least specific route possible, and thus will
only be used if a more specific route does not exist (hence “gateway of last
resort”).
To configure a default route:
RouterA(config)# ip route 0.0.0.0 0.0.0.0 172.17.1.2
Advanced default routing is covered in great detail in another guide.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
62 Section 8
- Default Routing -
Default Routing
Normally, if a specific route to a particular network does not exist, a router
will drop all traffic destined to that network. A default route, or gateway of
last resort, allows traffic to be forwarded, even without a specific route to a
particular network.
The default route is identified by all zeros in both the network and subnet
mask (0.0.0.0 0.0.0.0). It is the least specific route possible, and thus will
only be used if a more specific route does not exist (hence “gateway of last
resort”).
To configure a default route:
Router(config)# ip route 0.0.0.0 0.0.0.0 172.17.1.2
It is possible to specify an entire default network on a Cisco device:
Router(config)# ip default-network 172.20.0.0
The 172.20.0.0 network must already exist in the routing table (either
statically or dynamically), and will be marked as the gateway of last resort.
If IP routing is disabled on a Cisco IOS device, the following command will
configure a default-gateway:
Router(config)# no ip routing
Router(config)# ip default-gateway 192.168.1.1
Essentially, the Cisco router will act as a host device, and will perform no
routing functions on behalf of other hosts. The router will simply forward its
own locally-originated traffic to the default-gateway, assuming that traffic is
destined for a remote network.
It is possible to generate a default route in most routing protocols (RIP,
OSPF, IS-IS, & BGP) using the default-information originate command:
Router(config)# router rip Router(config-router)# default-information originate (Reference: http://www.cisco.com/warp/public/105/default.html)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
63 ________________________________________________ Part III Dynamic Routing Protocols ________________________________________________
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64 Section 9
- Routing Information Protocol -
RIP (Routing Information Protocol)
RIP is a standardized Distance Vector protocol, designed for use on smaller
networks. RIP was one of the first true Distance Vector routing protocols,
and is supported on a wide variety of systems.
RIP adheres to the following Distance Vector characteristics:
• RIP sends out periodic routing updates (every 30 seconds)
• RIP sends out the full routing table every periodic update
• RIP uses a form of distance as its metric (in this case, hopcount)
• RIP uses the Bellman-Ford Distance Vector algorithm to determine
the best “path” to a particular destination
Other characteristics of RIP include:
• RIP supports IP and IPX routing.
• RIP utilizes UDP port 520
• RIP routes have an administrative distance of 120.
• RIP has a maximum hopcount of 15 hops.
Any network that is 16 hops away or more is considered unreachable to RIP,
thus the maximum diameter of the network is 15 hops. A metric of 16 hops
in RIP is considered a poison route or infinity metric.
If multiple paths exist to a particular destination, RIP will load balance
between those paths (by default, up to 4) only if the metric (hopcount) is
equal. RIP uses a round-robin system of load-balancing between equal
metric routes, which can lead to pinhole congestion.
For example, two paths might exist to a particular destination, one going
through a 9600 baud link, the other via a T1. If the metric (hopcount) is
equal, RIP will load-balance, sending an equal amount of traffic down the
9600 baud link and the T1. This will (obviously) cause the slower link to
become congested.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
65 RIP Versions
RIP has two versions, Version 1 (RIPv1) and Version 2 (RIPv2).
RIPv1 (RFC 1058) is classful, and thus does not include the subnet mask
with its routing table updates. Because of this, RIPv1 does not support
Variable Length Subnet Masks (VLSMs). When using RIPv1, networks
must be contiguous, and subnets of a major network must be configured with
identical subnet masks. Otherwise, route table inconsistencies (or worse)
will occur.
RIPv1 sends updates as broadcasts to address 255.255.255.255.
RIPv2 (RFC 2543) is classless, and thus does include the subnet mask with
its routing table updates. RIPv2 fully supports VLSMs, allowing
discontiguous networks and varying subnet masks to exist.
Other enhancements offered by RIPv2 include:
• Routing updates are sent via multicast, using address 224.0.0.9
• Encrypted authentication can be configured between RIPv2 routers
• Route tagging is supported (explained in a later section)
RIPv2 can interoperate with RIPv1. By default:
• RIPv1 routers will sent only Version 1 packets
• RIPv1 routers will receive both Version 1 and 2 updates
• RIPv2 routers will both send and receive only Version 2 updates
We can control the version of RIP a particular interface will “send” or
“receive.”
Unless RIPv2 is manually specified, a Cisco will default to RIPv1 when
configuring RIP.
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66 RIPv1 Basic Configuration
Routing protocol configuration occurs in Global Configuration mode. On
Router A, to configure RIP, we would type:
Router(config)# router rip Router(config-router)# network 172.16.0.0 Router(config-router)# network 172.17.0.0
The first command, router rip, enables the RIP process.
The network statements tell RIP which networks you wish to advertise to
other RIP routers. We simply list the networks that are directly connected to
our router. Notice that we specify the networks at their classful boundaries,
and we do not specify a subnet mask.
To configure Router B:
Router(config)# router rip Router(config-router)# network 172.17.0.0 Router(config-router)# network 172.18.0.0
The routing table on Router A will look like:
RouterA# show ip route
<eliminated irrelevant header>
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Ethernet0
C 172.17.0.0 is directly connected, Serial0
R 172.18.0.0 [120/1] via 172.17.1.2, 00:00:00, Serial0
The routing table on Router B will look like:
RouterB# show ip route
<eliminated irrelevant header>
Gateway of last resort is not set
C 172.17.0.0 is directly connected, Serial0
C 172.18.0.0 is directly connected, Ethernet0
R 172.16.0.0 [120/1] via 172.17.1.1, 00:00:00, Serial0
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
67 Limitations of RIPv1
The example on the previous page works fine with RIPv1, because the
networks are contiguous and the subnet masks are consistent. Consider the
following example:
This particular scenario will still work when using RIPv1, despite the fact
that we’ve subnetted the major 10.0.0.0 network. Notice that the subnets are
contiguous (that is, they belong to the same major network), and use the
same subnet mask.
When Router A sends a RIPv1 update to Router B via Serial0, it will not
include the subnet mask for the 10.1.0.0 network. However, because the
10.3.0.0 network is in the same major network as the 10.1.0.0 network, it
will not summarize the address. The route entry in the update will simply
state “10.1.0.0”.
Router B will accept this routing update, and realize that the interface
receiving the update (Serial0) belongs to the same major network as the
route entry of 10.1.0.0. It will then apply the subnet mask of its Serial0
interface to this route entry.
Router C will similarly send an entry for the 10.2.0.0 network to Router B.
Router B’s routing table will thus look like:
RouterB# show ip route
Gateway of last resort is not set
10.0.0.0/16 is subnetted, 4 subnets
C 10.3.0.0 is directly connected, Serial0
C 10.4.0.0 is directly connected, Serial1
R 10.1.0.0 [120/1] via 10.3.5.1, 00:00:00, Serial0
R 10.2.0.0 [120/1] via 10.4.5.1, 00:00:00, Serial1
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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68
Limitations of RIPv1 (continued)
Consider the following, slightly altered, example:
We’ll assume that RIPv1 is configured correctly on all routers. Notice that
our networks are no longer contiguous. Both Router A and Router C contain
subnets of the 10.0.0.0 major network (10.1.0.0 and 10.2.0.0 respectively).
Separating these networks now are two Class C subnets (192.168.123.0 and
192.168.111.0).
Why is this a problem? Again, when Router A sends a RIPv1 update to
Router B via Serial, it will not include the subnet mask for the 10.1.0.0
network. Instead, Router A will consider itself a border router, as the
10.1.0.0 and 192.168.123.0 networks do not belong to the same major
network. Router A will summarize the 10.1.0.0/16 network to its classful
boundary of 10.0.0.0/8.
Router B will accept this routing update, and realize that it does not have a
directly connected interface in the 10.x.x.x scheme. Thus, it has no subnet
mask to apply to this route. Because of this, Router B will install the
summarized 10.0.0.0 route into its routing table.
Router C, similarly, will consider itself a border router between networks
10.2.0.0 and 192.168.111.0. Thus, Router C will also send a summarized
10.0.0.0 route to Router B.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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69
Limitations of RIPv1 (continued)
Router B’s routing table will then look like:
RouterB# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
C 192.168.111.0 is directly connected, Serial1
R 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0
[120/1] via 192.168.111.2, 00:00:00, Serial1
That’s right, Router B now has two equal metric routes to get to the
summarized 10.0.0.0 network, one through Router A and the other through
Router C. Router B will now load balance all traffic to any 10.x.x.x network
between routers A and C. Suffice to say, this is not a good thing.
It gets better. Router B then tries to send routing updates to Router A and
Router C, including the summary route of 10.0.0.0/8. Router A’s routing
table looks like:
RouterA# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
10.0.0.0/16 is subnetted, 1 subnet
C 10.1.0.0 is directly connected, Ethernet0
Router A will receive the summarized 10.0.0.0/8 route from Router B, and
will reject it. This is because it already has the summary network of 10.0.0.0
in its routing table, and it’s directly connected. Router C will respond
exactly the same, and the 10.1.0.0/16 and 10.2.0.0/16 networks will never be
able to communicate.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
70 RIPv2 Configuration
RIPv2 overcomes the limitations of RIPv1 by including the subnet mask in
its routing updates. By default, Cisco routers will use RIPv1. To change to
Version 2, you must type:
Router(config)# router rip Router(config-router)# version 2
Thus, the configuration of Router A would be:
RouterA(config)# router rip RouterA(config-router)# version 2 RouterA(config-router)# network 10.0.0.0 RouterA(config-router)# network 192.168.123.0
Despite the fact that RIPv2 is a classless routing protocol, we still specify
networks at their classful boundaries, without a subnet mask.
However, when Router A sends a RIPv2 update to Router B via Serial0, by
default it will still summarize the 10.1.0.0/16 network to 10.0.0.0/8. Again,
this is because the 10.1.0.0 and 192.168.123.0 networks do not belong to the
same major network. Thus, RIPv2 acts like RIPv1 in this circumstance…
…unless you disable auto summarization:
RouterA(config)# router rip RouterA(config-router)# version 2 RouterA(config-router)# no auto-summary
The no auto-summary command will prevent Router A from summarizing
the 10.1.0.0 network. Instead, Router A will send an update that includes
both the subnetted network (10.1.0.0) and its subnet mask (255.255.0.0).
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
71 RIP Timers
RIP has four basic timers:
Update Timer (default 30 seconds) – indicates how often the router will
send out a routing table update.
Invalid Timer (default 180 seconds) – indicates how long a route will
remain in a routing table before being marked as invalid, if no new updates
are heard about this route. The invalid timer will be reset if an update is
received for that particular route before the timer expires.
A route marked as invalid is not immediately removed from the routing
table. Instead, the route is marked (and advertised) with a metric of 16,
indicating it is unreachable, and placed in a hold-down state.
Hold-down Timer (default 180 seconds) – indicates how long RIP will
“suppress” a route that it has placed in a hold-down state. RIP will not
accept any new updates for routes in a hold-down state, until the hold-down
timer expires.
A route will enter a hold-down state for one of three reasons:
• The invalid timer has expired.
• An update has been received from another router, marking that route
with a metric of 16 (unreachable).
• An update has been received from another router, marking that route
with a higher metric than what is currently in the routing table (this is
to prevent loops).
Flush Timer (default 240 seconds) – indicates how long a route can remain
in a routing table before being flushed, if no new updates are heard about
this route. The flush timer runs concurrently with the hold-down timer, and
thus will flush out a route 60 seconds after it has been marked invalid.
RIP timers must be identical on all routers on the RIP network, otherwise
massive instability will occur.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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72
RIP Timers Configuration and Example
Consider the above example. Router A receives a RIP update from Router B
that includes network 172.18.0.0. Router A adds this network to its routing
table:
RouterA# show ip route
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Ethernet0
C 172.17.0.0 is directly connected, Serial0
R 172.18.0.0 [120/1] via 172.17.1.2, 00:00:00, Serial0
Immediately, Router A sets an invalid timer of 180 seconds. If no update for
this route is heard for 180 seconds, several things will occur:
• The route is marked as invalid in the routing table.
• The route enters a hold-down state (triggering the hold-down timer).
• The flush timer is triggered.
• The route is advertised to all other routers as unreachable.
The hold-down timer runs for 180 seconds after the route is marked as
invalid. The router will not accept any new updates for this route until this
hold-down period expires. The flush timer runs for 240 seconds after the
route is marked as invalid.
If no update is heard at all, the route will be deleted completely once the
flush timer expires. By default, this will be 60 seconds after the hold-down
timer expires (240 – 180 seconds = 60 seconds). Remember, the hold-down
and flush timers run concurrently.
To configure the RIP timers:
Router(config)# router rip
Router(config-router)# timers basic 20 120 120 160
The timers basic command allows us to change the update (20), invalid
(120), hold-down (120), and flush (240) timers. To return the timers back to
their defaults:
Router(config-router)# no timers basic
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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73
RIP Loop Avoidance Mechanisms
RIP, as a Distance Vector routing protocol, is susceptible to loops.
Let’s assume no loop avoidance mechanisms are configured on either router.
If the 172.18.0.0 network fails, Router B will send out an update to Router A
within 30 seconds (whenever its update timer expires) stating that route is
unreachable (metric = 16).
But what if an update from Router A reaches Router B before this can
happen? Router A believes it can reach the 172.18.0.0 network in one hop
(through Router B). This will cause Router B to believe it can reach the
failed 172.18.0.0 network in two hops, through Router A. Both routers will
continue to increment the metric for the network until they reach a hop count
of 16, which is unreachable. This behavior is known as counting to infinity.
How can we prevent this from happening? There are several loop avoidance
mechanisms:
Split-Horizon – Prevents a routing update from being sent out the interface
it was received on. In our above example, this would prevent Router A from
sending an update for the 172.18.0.0 network back to Router B, as it
originally learned the route from Router B. Split-horizon is enabled by
default on Cisco Routers.
Route-Poisoning – Works in conjunction with split-horizon, by triggering
an automatic update for the failed network, without waiting for the update
timer to expire. This update is sent out all interfaces with an infinity metric
for that network.
Hold-Down Timers – Prevents RIP from accepting any new updates for
routes in a hold-down state, until the hold-down timer expires. If Router A
sends an update to Router B with a higher metric than what is currently in
Router B’s routing table, that route will be placed in a hold-down state.
(Router A’s metric for the 172.18.0.0 network is 1; while Router B’s metric
is 0).
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
74 RIP Passive Interfaces
It is possible to control which router interfaces will participate in the RIP
process.
Consider the following scenario. Router C does not want to participate in the
RIP domain. However, it still wants to listen to updates being sent from
Router B, just not send any updates back to Router B:
RouterC(config)# router rip RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface s0
The passive-interface command will prevent updates from being sent out of
the Serial0 interface, but Router C will still receive updates on this interface.
We can configure all interfaces to be passive using the passive-interface
default command, and then individually use the no passive-interface
command on the interfaces we do want updates to be sent out:
RouterC(config)# router rip RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface default
RouterC(config-router)# no passive-interface e0
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
75 RIP Neighbors
Recall that RIPv1 sends out its updates as broadcasts, whereas RIPv2 sends
out its updates as multicasts to the 224.0.0.9 address. We can configure
specific RIP neighbor commands, which will allow us to unicast routing
updates to those neighbors.
On Router B: RouterB(config)# router rip RouterB(config-router)# network 10.3.0.0 RouterB(config-router)# network 10.4.0.0 RouterB(config-router)# neighbor 10.3.5.1 RouterB(config-router)# neighbor 10.4.5.1
Router B will now unicast RIP updates to Router A and Router C.
However, Router B will still broadcast (if RIPv1) or multicast (if RIPv2) its
updates, in addition to sending unicast updates to its neighbors. In order to
prevent broadcast/multicast updates, we must also use passive interfaces:
RouterB(config)# router rip RouterB(config-router)# passive-interface s0 RouterB(config-router)# passive-interface s1 RouterB(config-router)# neighbor 10.3.5.1 RouterB(config-router)# neighbor 10.4.5.1
The passive-interface commands prevent the updates from being
broadcasted or multicasted. The neighbor commands still allow unicast
updates to those specific neighbors.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
76 RIPv2 Authentication
RIPv2 supports authentication to secure routing updates.
The first step is creating a shared authentication key that must be identical on
both routers. This is accomplished in global configuration mode:
RouterA(config)# key chain MYCHAIN
RouterA(config-keychain)# key 1 RouterA(config-keychain-key)# key-string MYPASSWORD
RouterB(config)# key chain MYCHAIN
RouterB(config-keychain)# key 1 RouterB(config-keychain-key)# key-string MYPASSWORD
The first command creates a key chain called MYCHAIN. We must then
associate a key to our keychain. Then we actually configure the shared key
using the key-string command.
We then apply our key chain to the interface connecting to the other router:
RouterA(config)# interface s0
RouterA(config-if)# ip rip authentication key-chain MYCHAIN
RouterB(config)# interface s0
RouterB(config-if)# ip rip authentication key-chain MYCHAIN
If there was another router off of Router B’s Ethernet port, we could create a
separate key chain with a different key-string. Every router on the RIP
domain does not need to use the same key chain, only interfaces directly
connecting two (or more) routers.
The final step in configuring authentication is identifying which encryption
to use. By default, the key is sent in clear text:
RouterA(config)# interface s0
RouterA(config-if)# ip rip authentication mode text
Or we can use MD5 encryption for additional security:
RouterA(config)# interface s0
RouterA(config-if)# ip rip authentication mode md5
Whether text or MD5 is used, it must be the same on both routers.
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
77 Altering RIP’s Metric
Consider the above example. Router B has two paths to get to the
192.168.100.0 network, via Router A and Router C. Because the metric is
equal (1 hop), Router B will load balance between these two paths.
What if we wanted Router B to only go through Router A, and use Router C
only as a backup? To accomplish this, we can adjust RIP’s metric to make
one route more preferred than the other.
The first step is creating an access-list on Router B that defines which route
we wish to alter:
RouterB(config)# ip access-list standard MYLIST
RouterB(config-std-nacl)# permit 192.168.100.0 0.0.0.255
Next, we tell RIP how much to offset this route if received by Router C:
RouterB(config)# router rip
RouterB(config-router)# offset-list MYLIST in 4 s1
We specify an offset-list pointing to our access list named MYLIST. We will
increase the routing metric by 4 for that route coming inbound to interface
Serial 1.
Thus, when Router C sends an update to Router C for the 192.168.100.0
network, Router B will increase its metric of 1 hop to 5 hops, thus making
Router A’s route preferred.
We could have also configured Router C to advertise that route with a
higher metric (notice the out in the offset-list command):
RouterC(config)# ip access-list standard MYLIST
RouterC(config-std-nacl)# permit 192.168.100.0 0.0.0.255
RouterC(config)# router rip
RouterC(config-router)# offset-list MYLIST out 4 s0
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
78
Interoperating between RIPv1 and RIPv2
Recall that, with some configuration, RIPv1 and RIPv2 can interoperate. By
default:
• RIPv1 routers will sent only Version 1 packets
• RIPv1 routers will receive both Version 1 and 2 updates
• RIPv2 routers will both send and receive only Version 2 updates
If Router A is running RIP v1, and Router B is running RIP v2, some
additional configuration is necessary.
Either we must configure Router A to send Version 2 updates:
RouterA(config)# interface s0
RouterA(config-if)# ip rip send version 2
Or configure Router B to accept Version 1 updates.
RouterB(config)# interface s0
RouterB(config-if)# ip rip receive version 1
Notice that this is configured on an interface. Essentially, we’re configuring
the version of RIP on a per-interface basis.
We can also have an interface send or receive both versions simultaneously:
RouterB(config)# interface s0
RouterB(config-if)# ip rip receive version 1 2
We can further for RIPv2 to send broadcast updates, instead of multicasts:
RouterB(config)# interface s0
RouterB(config)# ip rip v2-broadcast
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
79 Triggering RIP Updates
On point-to-point interfaces, we can actually force RIP to only send routing
updates if there is a change:
RouterB(config)# interface s0.150 point-to-point
RouterB(config-if)# ip rip triggered
Again, this is only applicable to point-to-point links. We cannot configure
RIP triggered updates on an Ethernet network.
Troubleshooting RIP
Various troubleshooting commands exist for RIP.
To view the IP routing table:
Router# show ip route
<eliminated irrelevant header>
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Ethernet0
C 172.17.0.0 is directly connected, Serial0
R 172.18.0.0 [120/1] via 172.17.1.2, 00:00:15, Serial0
R 192.168.123.0 [120/1] via 172.16.1.1, 00:00:00, Ethernet0
To view a specific route within the IP routing table:
Router# show ip route 172.18.0.0
Routing entry for 172.18.0.0/16
Known via “rip”, distance 120, metric 1
Last update from 172.17.1.2 on Serial 0, 00:00:15 ago
To debug RIP in real time:
Router# debug ip rip
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
80 Troubleshooting RIP (continued)
To view information specific to the RIP protocol:
Router# show ip protocols
Routing Protocol is "rip"
Sending updates every 30 seconds, next due in 20 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Incoming routes will have 4 added to metric if on list 1
Redistributing: connected, static, rip
Default version control: send version 1, receive any version
Interface Send Recv Triggered RIP Key-chain
Ethernet0 1 1 2
Serial0 1 2 1 2
Automatic network summarization is in effect
Maximum path: 4 Routing for Networks: 172.16.0.0 172.17.0.0 Routing Information Sources:
Gateway Distance Last Update
172.17.1.2 120 00:00:17
Distance: (default is 120)
This command provides us with information on RIP timers, on the RIP
versions configured on each interface, and the specific networks RIP is
advertising.
To view all routes in the RIP database, and not just the entries added to the
routing table:
Router# show ip rip database
7.0.0.0/8 auto-summary 7.0.0.0/8
[5] via 172.16.1.1, 00:00:06, Ethernet0
172.16.0.0/16 directly connected, Ethernet0
172.17.0.0/16 directly connected, Serial0
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81 Section 10
- Interior Gateway Routing Protocol -
IGRP (Interior Gateway Routing Protocol)
IGRP is a Cisco-proprietary Distance-Vector protocol, designed to be more
scalable than RIP, its standardized counterpart.
IGRP adheres to the following Distance-Vector characteristics:
• IGRP sends out periodic routing updates (every 90 seconds).
• IGRP sends out the full routing table every periodic update.
• IGRP uses a form of distance as its metric (in this case, a composite of
bandwidth and delay).
• IGRP uses the Bellman-Ford Distance Vector algorithm to determine
the best “path” to a particular destination.
Other characteristics of IGRP include:
• IGRP supports only IP routing.
• IGRP utilizes IP protocol 9.
• IGRP routes have an administrative distance of 100.
• IGRP, by default, supports a maximum of 100 hops. This value can
be adjusted to a maximum of 255 hops.
• IGRP is a classful routing protocol.
IGRP uses Bandwidth and Delay of the Line, by default, to calculate its
distance metric. Reliability, Load, and MTU are optional attributes that can
be used to calculate the distance metric.
IGRP requires that you include an Autonomous System (AS) number in its
configuration. Only routers in the same Autonomous system will send
updates between each other.
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82 Configuring IGRP
Routing protocol configuration occurs in Global Configuration mode. On
Router A, to configure IGRP, we would type:
Router(config)# router igrp 10
Router(config-router)# network 172.16.0.0 Router(config-router)# network 172.17.0.0
The first command, router igrp 10, enables the IGRP process. The “10”
indicates the Autonomous System number that we are using. Only other
IGRP routers in Autonomous System 10 will share updates with this router.
The network statements tell IGRP which networks you wish to advertise to
other RIP routers. We simply list the networks that are directly connected to
our router. Notice that we specify the networks at their classful boundaries,
and we do not specify a subnet mask.
To configure Router B:
Router(config)# router igrp 10
Router(config-router)# network 172.17.0.0 Router(config-router)# network 172.18.0.0
The routing table on Router A will look like:
RouterA# show ip route
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Ethernet0
C 172.17.0.0 is directly connected, Serial0
I 172.18.0.0 [120/1] via 172.17.1.2, 00:00:00, Serial0
The routing table on Router B will look like:
RouterB# show ip route
Gateway of last resort is not set
C 172.17.0.0 is directly connected, Serial0
C 172.18.0.0 is directly connected, Ethernet0
I 172.16.0.0 [120/1] via 172.17.1.1, 00:00:00, Serial0
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83 Limitations of IGRP
The example on the previous page works fine with IGRP, because the
networks are contiguous and the subnet masks are consistent. Consider the
following example:
This particular scenario will still work when using IGRP, despite the fact
that we’ve subnetted the major 10.0.0.0 network. Notice that the subnets are
contiguous (that is, they belong to the same major network), and use the
same subnet mask.
When Router A sends an IGRP update to Router B via Serial0, it will not
include the subnet mask for the 10.1.0.0 network. However, because the
10.3.0.0 network is in the same major network as the 10.1.0.0 network, it
will not summarize the address. The route entry in the update will simply
state “10.1.0.0”.
Router B will accept this routing update, and realize that the interface
receiving the update (Serial0) belongs to the same major network as the
route entry of 10.1.0.0. It will then apply the subnet mask of its Serial0
interface to this route entry.
Router C will similarly send an entry for the 10.2.0.0 network to Router B.
Router B’s routing table will thus look like:
RouterB# show ip route
Gateway of last resort is not set
10.0.0.0/16 is subnetted, 4 subnets
C 10.3.0.0 is directly connected, Serial0
C 10.4.0.0 is directly connected, Serial1
I 10.1.0.0 [120/1] via 10.3.5.1, 00:00:00, Serial0
I 10.2.0.0 [120/1] via 10.4.5.1, 00:00:00, Serial1
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84
Limitations of IGRP (continued)
Consider the following, slightly altered, example:
We’ll assume that IGRP is configured correctly on all routers. Notice that
our networks are no longer contiguous. Both Router A and Router C contain
subnets of the 10.0.0.0 major network (10.1.0.0 and 10.2.0.0 respectively).
Separating these networks now are two Class C subnets (192.168.123.0 and
192.168.111.0).
Why is this a problem? Again, when Router A sends an IGRP update to
Router B via Serial, it will not include the subnet mask for the 10.1.0.0
network. Instead, Router A will consider itself a border router, as the
10.1.0.0 and 192.168.123.0 networks do not belong to the same major
network. Router A will summarize the 10.1.0.0/16 network to its classful
boundary of 10.0.0.0/8.
Router B will accept this routing update, and realize that it does not have a
directly connected interface in the 10.x.x.x scheme. Thus, it has no subnet
mask to apply to this route. Because of this, Router B will install the
summarized 10.0.0.0 route into its routing table.
Router C, similarly, will consider itself a border router between networks
10.2.0.0 and 192.168.111.0. Thus, Router C will also send a summarized
10.0.0.0 route to Router B.
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unless otherwise noted. All other material copyright © of their respective owners.
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85
Limitations of IGRP (continued)
Router B’s routing table will then look like:
RouterB# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
C 192.168.111.0 is directly connected, Serial1
I 10.0.0.0 [120/1] via 192.168.123.1, 00:00:00, Serial0
[120/1] via 192.168.111.2, 00:00:00, Serial1
That’s right, Router B now has two equal metric routes to get to the
summarized 10.0.0.0 network, one through Router A and the other through
Router C. Router B will now load balance all traffic to any 10.x.x.x network
between routers A and C. Suffice to say, this is not a good thing.
It gets better. Router B then tries to send routing updates to Router A and
Router C, including the summary route of 10.0.0.0/8. Router A’s routing
table looks like:
RouterA# show ip route
Gateway of last resort is not set
C 192.168.123.0 is directly connected, Serial0
10.0.0.0/16 is subnetted, 1 subnet
C 10.1.0.0 is directly connected, Ethernet0
Router A will receive the summarized 10.0.0.0/8 route from Router B, and
will reject it. This is because it already has the summary network of 10.0.0.0
in its routing table, and it’s directly connected. Router C will respond
exactly the same, and the 10.1.0.0/16 and 10.2.0.0/16 networks will never be
able to communicate.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
86 IGRP Timers
IGRP has four basic timers:
Update Timer (default 90 seconds) – indicates how often the router will
send out a routing table update.
Invalid Timer (default 270 seconds) – indicates how long a route will
remain in a routing table before being marked as invalid, if no new updates
are heard about this route. The invalid timer will be reset if an update is
received for that particular route before the timer expires.
A route marked as invalid is not immediately removed from the routing
table. Instead, the route is marked (and advertised) with a metric of 101
(remember, 100 maximum hops is default), indicating it is unreachable, and
placed in a hold-down state.
Hold-down Timer (default 280 seconds) – indicates how long IGRP will
“suppress” a route that it has placed in a hold-down state. IGRP will not
accept any new updates for routes in a hold-down state, until the hold-down
timer expires.
A route will enter a hold-down state for one of three reasons:
• The invalid timer has expired.
• An update has been received from another router, marking that route
with a metric of 101 (unreachable).
• An update has been received from another router, marking that route
with a higher metric than what is currently in the routing table (this is
to prevent loops).
Flush Timer (default 630 seconds) – indicates how long a route can remain
in a routing table before being flushed, if no new updates are heard about
this route. The flush timer runs concurrently with the hold-down timer, and
thus will flush out a route 350 seconds after it has been marked invalid.
IGRP timers must be identical on all routers on the IGRP network, otherwise
massive instability will occur.
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87
IGRP Loop Avoidance Mechanisms
IGRP, as a Distance Vector routing protocol, is susceptible to loops.
Let’s assume no loop avoidance mechanisms are configured on either router.
If the 172.18.0.0 network fails, Router B will send out an update to Router A
within 30 seconds (whenever its update timer expires) stating that route is
unreachable.
But what if an update from Router A reaches Router B before this can
happen? Router A believes it can reach the 172.18.0.0 network in one hop
(through Router B). This will cause Router B to believe it can reach the
failed 172.18.0.0 network in two hops, through Router A. Both routers will
continue to increment the metric for the network until they reach an infinity
hop count (by default, 101). This behavior is known as counting to infinity.
How can we prevent this from happening? There are several loop avoidance
mechanisms:
Split-Horizon – Prevents a routing update from being sent out the interface
it was received on. In our above example, this would prevent Router A from
sending an update for the 172.18.0.0 network back to Router B, as it
originally learned the route from Router B. Split-horizon is enabled by
default on Cisco Routers.
Route-Poisoning – Works in conjunction with split-horizon, by triggering
an automatic update for the failed network, without waiting for the update
timer to expire. This update is sent out all interfaces with an infinity metric
for that network.
Hold-Down Timers – Prevents IGRP from accepting any new updates for
routes in a hold-down state, until the hold-down timer expires. If Router A
sends an update to Router B with a higher metric than what is currently in
Router B’s routing table, that route will be placed in a hold-down state.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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88 IGRP Passive Interfaces
It is possible to control which router interfaces will participate in the IGRP
process.
Consider the following scenario. Router C does not want to participate in the
IGRP domain. However, it still wants to listen to updates being sent from
Router B, just not send any updates back to Router B:
RouterC(config)# router igrp 10
RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface s0
The passive-interface command will prevent updates from being sent out of
the Serial0 interface, but Router C will still receive updates on this interface.
We can configure all interfaces to be passive using the passive-interface
default command, and then individually use the no passive-interface
command on the interfaces we do want updates to be sent out:
RouterC(config)# router igrp 10
RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface default
RouterC(config-router)# no passive-interface e0
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
89 Advanced IGRP Configuration
To change the maximum hop-count to 255 for IGRP:
Router(config)# router igrp 10
Router(config-router)# metric maximum-hops 255
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90 Section 11
- Enhanced Interior Gateway Routing Protocol -
EIGRP (Enhanced Interior Gateway Routing Protocol)
EIGRP is a Cisco-proprietary Hybrid routing protocol, incorporating
features of both Distance-Vector and Link-State routing protocols.
EIGRP adheres to the following Hybrid characteristics:
• EIGRP uses Diffusing Update Algorithm (DUAL) to determine the
best path among all “feasible” paths. DUAL also helps ensure a loopfree
routing environment.
• EIGRP will form neighbor relationships with adjacent routers in the
same Autonomous System (AS).
• EIGRP traffic is either sent as unicasts, or as multicasts on address
224.0.0.10, depending on the EIGRP packet type.
• Reliable Transport Protocol (RTP) is used to ensure delivery of most
EIGRP packets.
• EIGRP routers do not send periodic, full-table routing updates.
Updates are sent when a change occurs, and include only the change.
• EIGRP is a classless protocol, and thus supports VLSMs.
Other characteristics of EIGRP include:
• EIGRP supports IP, IPX, and Appletalk routing.
• EIGRP applies an Administrative Distance of 90 for routes originating
within the local Autonomous System.
• EIGRP applies an Administrative Distance of 170 for external routes
coming from outside the local Autonomous System
• EIGRP uses Bandwidth and Delay of the Line, by default, to
calculate its distance metric. It also supports three other parameters to
calculate its metric: Reliability, Load, and MTU.
• EIGRP has a maximum hop-count of 224, though the default
maximum hop-count is set to 100.
EIGRP, much like OSPF, builds three separate tables:
• Neighbor table – list of all neighboring routers. Neighbors must
belong to the same Autonomous System
• Topology table – list of all routes in the Autonomous System
• Routing table – contains the best route for each known network
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
91 EIGRP Neighbors
EIGRP forms neighbor relationships, called adjacencies, with other routers
in the same AS by exchanging Hello packets. Only after an adjacency is
formed can routers share routing information. Hello packets are sent as
multicasts to address 224.0.0.10.
By default, on LAN and high-speed WAN interfaces, EIGRP Hellos are sent
every 5 seconds. On slower WAN links (T1 speed or slower), EIGRP Hellos
are sent every 60 seconds by default.
The EIGRP Hello timer can be adjusted on a per interface basis:
Router(config-if)# ip hello-interval eigrp 10 7
The above command allows us to change the hello timer to 7 seconds for
Autonomous System 10.
In addition to the Hello timer, EIGRP neighbors are stamped with a Hold
timer. The Hold timer indicates how long a router should wait before
marking a neighbor inactive, if it stops receiving hello packets from that
neighbor.
By default, the Hold timer is three times the Hello timer. Thus, on highspeed
links the timer is set to 15 seconds, and on slower WAN links the
timer is set to 180 seconds.
The Hold timer can also be adjusted on a per interface basis:
Router(config-if)# ip hold-interval eigrp 10 21
The above command allows us to change the hold timer to 21 seconds for
Autonomous System 10.
Changing the Hello timer does not automatically change the Hold timer.
Additionally, Hello and Hold timers do not need to match between routers
for an EIGRP neighbor relationship to form.
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#eigrp_work)
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
92 EIGRP Neighbors (continued)
A neighbor table is constructed from the EIGRP Hello packets, which
includes the following information:
• The IP address of the neighboring router.
• The local interface that received the neighbor’s Hello packet.
• The Hold timer.
• A sequence number indicating the order neighbors were learned.
Adjacencies will not form unless the primary IP addresses on connecting
interfaces are on the same subnet. Neighbors cannot be formed on secondary
addresses.
If connecting interfaces are on different subnets, an EIGRP router will log
the following error to console when a multicast Hello is received:
00:11:22: IP-EIGRP: Neighbor 172.16.1.1 not on common
subnet for Serial0
Always ensure that primary IP addresses belong to the same subnet between
EIGRP neighbors.
To log all neighbor messages and errors to console, use the following two
commands:
Router(config)# router eigrp 10
Router(config-router)# eigrp log-neighbor-changes Router(config-router)# eigrp log-neighbor-warnings (Reference: http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080093f09.shtml)
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93
The EIGRP Topology Table
Once EIGRP neighbors form adjacencies, they will begin to share routing
information. Each router’s update contains a list of all routes known by that
router, and the respective metrics for those routes.
All such routes are added to an EIGRP router’s topology table. The route
with the lowest metric to each network will become the Feasible Distance
(FD). The Feasible Distance for each network will be installed into the
routing table.
The Feasible Distance is derived from the Advertised Distance of the router
sending the update, and the local router’s metric to the advertising router.
Confused? Consider the following example:
Router A has three separate paths to the Destination Network, either through
Router B, C, or D. If we add up the metrics to form a “distance” (the metrics
are greatly simplified in this example), we can determine the following:
• Router B’s Feasible Distance to the Destination Network is 8.
• Router C’s Feasible Distance to the Destination Network is 23.
• Router D’s Feasible Distance to the Destination Network is 9.
Router B sends an update to Router A, it will provide an Advertised
Distance of 8 to the Destination Network. Router C will provide an AD of
23, and D will provide an AD of 9.
Router A calculates the total distance to the Destination network by adding
the AD of the advertising router, with its own distance to reach that
advertising router. For example, Router A’s metric to Router B is 8; thus, the
total distance will be 16 to reach the Destination Network through Router B.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
94
The EIGRP Topology Table (continued)
Remember, however, that Router A’s Feasible Distance must be the route
with the lowest metric. If we add the Advertised Distance with the local
metric between each router, we would see that:
• The route through Router B has a distance of 16 to the destination
• The route through Router C has a distance of 27 to the destination
• The route through Router D has a distance of 11 to the destination
Thus, the route through Router D (metric of 11) would become the Feasible
Distance for Router A, and is added to the routing table as the best route.
This route is identified as the Successor.
To allow convergence to occur quickly if a link fails, EIGRP includes
backup routes in the topology table called Feasible Successors (FS). A
route will only become a Successor if its Advertised Distance is less than the
current Feasible Distance. This is known as a Feasible Condition (FC).
For example, we determined that Router A’s Feasible Distance to the
destination is 11, through Router D. Router C’s Advertised Distance is 23,
and thus would not become a Feasible Successor, as it has a higher metric
than Router A’s current Feasible Distance. Routes that are not Feasible
Successors become route Possibilities.
Router B’s Advertised Distance is 8, which is less than Router A’s current
Feasible Distance. Thus, the route through Router B to the Destination
Network would become a Feasible Successor.
Feasible Successors provide EIGRP with redundancy, without forcing
routers to re-converge (thus stopping the flow of traffic) when a topology
change occurs. If no Feasible Successor exists and a link fails, a route will
enter an Active (converging) state until an alternate route is found.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
95 EIGRP Packet Types
EIGRP employs five packet types:
• Hello packets - multicast
• Update packets – unicast or multicast
• Query packets – multicast
• Reply packets – unicast
• Acknowledgement packets - unicast
Hello packets are used to form neighbor relationships, and were explained
in detail previously. Hello packets are always multicast to address
224.0.0.10.
Update packets are sent between neighbors to build the topology and
routing tables. Updates sent to new neighbors are sent as unicasts. However,
if a route’s metric is changed, the update is sent out as a multicast to address
224.0.0.10.
Query packets are sent by a router when a Successor route fails, and there
are no Feasible Successors in the topology table. The router places the route
in an Active state, and queries its neighbors for an alternative route. Query
packets are sent as a multicast to address 224.0.0.10.
Reply packets are sent in response to Query packets, assuming the
responding router has an alternative route (feasible successor). Reply
packets are sent as a unicast to the querying router.
Recall that EIGRP utilizes the Reliable Transport Protocol (RTP) to
ensure reliable delivery of most EIGRP packets. Delivery is guaranteed by
having packets acknowledged using…..Acknowledgment packets!
Acknowledgment packets (also known as ACK’s) are simply Hello packets
with no data, other than an acknowledgment number. ACK’s are always sent
as unicasts. The following packet types employ RTP to ensure reliable
delivery via ACK’s: • Update Packets • Query Packets • Reply Packets
Hello and Acknowledgments (ha!) packets do not utilize RTP, and thus do
not require acknowledgement.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
96 EIGRP Route States
An EIGRP route can exist in one of two states, in the topology table:
• Active state • Passive State
A Passive state indicates that a route is reachable, and that EIGRP is fully
converged. A stable EIGRP network will have all routes in a Passive state.
A route is placed in an Active state when the Successor and any Feasible
Successors fail, forcing the EIGRP to send out Query packets and reconverge.
Multiple routes in an Active state indicate an unstable EIGRP
network. If a Feasible Successor exists, a route should never enter an Active
state.
Routes will become Stuck-in-Active (SIA) when a router sends out a Query
packet, but does not receive a Reply packet within three minutes. In other
words, a route will become SIA if EIGRP fails to re-converge. The local
router will clear the neighbor adjacency with any router(s) that has failed to
Reply, and will place all routes from that neighbor(s) in an Active state.
To view the current state of routes in the EIGRP topology table:
Router# show ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(172.19.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.3.0.0/16, 1 successors, FD is 2297856
via 172.16.1.2 (2297856/128256), Serial0
P 172.19.0.0/16, 1 successors, FD is 281600
via Connected, Serial 1
To view only active routes in the topology table:
Router# show ip eigrp topology active
IP-EIGRP Topology Table for AS(10)/ID(172.19.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status
A 172.19.0.0/16, 1 successors, FD is 23456056 1 replies,
active 0:00:38, query-origin: Multiple Origins
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008010f016.shtml)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
97 EIGRP Metrics
EIGRP can utilize 5 separate metrics to determine the best route to a
destination:
• Bandwidth (K1) – Slowest link in the route path, measured in kilobits
• Load (K2) – Cumulative load of all outgoing interfaces in the path,
given as a fraction of 255
• Delay of the Line (K3) – Cumulative delay of all outgoing interfaces
in the path in tens of microseconds
• Reliability (K4) – Average reliability of all outgoing interfaces in the
path, given as a fraction of 255
• MTU (K5) – The smallest Maximum Transmission Unit in the path.
The MTU value is actually never used to calculate the metric
By default, only Bandwidth and Delay of the Line are used. This is
identical to IGRP, except that EIGRP provides a more granular metric by
multiplying the bandwidth and delay by 256. Bandwidth and delay are
determined by the interfaces that lead towards the destination network.
By default, the full formula for determining the EIGRP metric is:
[10000000/bandwidth + delay] * 256
The bandwidth value represents the link with the lowest bandwidth in the
path, in kilobits. The delay is the total delay of all outgoing interfaces in the
path.
As indicated above, each metric is symbolized with a “K” and then a
number. When configuring EIGRP metrics, we actually identify which
metrics we want EIGRP to consider. Again, by default, only Bandwidth and
Delay are considered. Thus, using on/off logic:
K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0
If all metrics were set to “on,” the full formula for determining the EIGRP
metric would be:
[K1 * bandwidth * 256 + (K2 * bandwidth) / (256 - load)
+ K3 * delay * 256] * [K5 / (reliability + K4)]
Remember, the “K” value is either set to on (“1”) or off (“0”).
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
98 Configuring EIGRP Metrics
EIGRP allows us to identify which metrics the protocol should consider,
using the following commands:
Router(config)# router eigrp 10
Router(config-router)# metric weights 0 1 1 1 0 0
The first command enables the EIGRP process for Autonomous System 10.
The second actually identifies which EIGRP metrics to use. The first number
(0) is for Type of Service, and should always be zero. The next numbers, in
order, are K1 (1), K2 (1), K3 (1), K4 (0), and K5 (0). Thus, we are
instructing EIGRP to use bandwidth, load, and delay to calculate the total
metric, but not reliability or MTU.
Our formula would thus be:
[K1 * bandwidth * 256 + (K2 * bandwidth) / (256 - load) + K3 * delay * 256]
The actual values of our metrics (such as bandwidth or delay) must be
configured indirectly. To adjust the bandwidth (in Kbps) of an interface:
Router(config)# int s0/0 Router(config-if)# bandwidth 64
Router(config-if)# ip bandwidth-percent eigrp 10 30
However, this command does not actually dictate the physical speed of the
interface. It merely controls how EIGRP considers this interface. Best
practice is to set the bandwidth to the actual physical speed of the interface.
By default, a serial interface will have a bandwidth of 1.544 Mbps (1544).
The ip bandwidth-percent eigrp command limits the percentage of
bandwidth EIGRP can use on an interface. The percentage is based on the
configured bandwidth value. By default, EIGRP will use up to 50% of the
bandwidth of an interface. The above command adjusts this to 30% for
Autonomous System 10. Percentages over 100% can be used.
If adjustments to the EIGRP metric need to be made, the delay metric (in
tens of microseconds) on an interface should be used:
Router(config)# int s0/0 Router(config-if)# delay 10000
Metric settings must be identical on the connecting interfaces of two
routers; otherwise they will not form a neighbor relationship.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
99 Configuring Basic EIGRP
Routing protocol configuration occurs in Global Configuration mode. On
Router A, to configure EIGRP, we would type:
RouterA(config)# router eigrp 10
RouterA(config-router)# network 172.16.0.0 RouterA(config-router)# network 10.0.0.0
The first command, router eigrp 10, enables the EIGRP process. The “10”
indicates the Autonomous System number that we are using. The
Autonomous System number can range from 1 to 65535.
Only other EIGRP routers in Autonomous System 10 will form neighbor
adjacencies and share updates with this router.
The network statements serve two purposes in EIGRP:
• First, they identify which networks you wish to advertise to other
EIGRP routers (similar to RIP).
• Second, they identify which interfaces on the local router to attempt to
form neighbor relationships out of (similar to OSPF).
Prior to IOS version 12.0(4), the network statements were classful, despite
the fact that EIGRP is a classless routing protocol. For example, the above
network 10.0.0.0 command would advertise the networks of directlyconnected
interfaces belonging to the 10.0.0.0/8 network and its subnets. It
would further attempt to form neighbor relationships out of these interfaces.
IOS version 12.0(4) and later provided us with more granular control of our
network statements. It introduced a wildcard mask parameter, which allows
us to choose the networks to advertise in a classless fashion:
RouterA(config)# router eigrp 10
RouterA(config-router)# network 172.16.0.0 0.0.255.255
RouterA(config-router)# network 10.1.4.0 0.0.0.255
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
100 EIGRP Passive Interfaces
It is possible to control which router interfaces will participate in the EIGRP
process. Just as with RIP, we can use the passive-interface command.
However, please note that the passive-interface command works differently
with EIGRP than with RIP or IGRP. EIGRP will no longer form neighbor
relationships out of a “passive” interface, thus this command prevents
updates from being sent or received out of this interface:
RouterC(config)# router eigrp 10
RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface s0
Router C will not form a neighbor adjacency with Router B.
We can configure all interfaces to be passive using the passive-interface
default command, and then individually use the no passive-interface
command on the interfaces we do want neighbors to be formed on:
RouterC(config)# router eigrp 10
RouterC(config-router)# network 10.4.0.0 RouterC(config-router)# network 10.2.0.0 RouterC(config-router)# passive-interface default
RouterC(config-router)# no passive-interface e0
Always remember, that the passive-interface command will prevent EIGRP
(and OSPF) from forming neighbor relationships out of that interface. No
routing updates are passed in either direction.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
101 EIGRP Auto-Summarization
EIGRP is a classless routing protocol that supports Variable Length Subnet
Masks (VLSMs). The above example would pose no problem for EIGRP.
However, EIGRP will still automatically summarize when crossing major
network boundaries.
For example, when Router A sends an EIGRP update to Router B via
Serial0, by default it will still summarize the 10.1.0.0/16 network to
10.0.0.0/8. This is because the 10.1.0.0/16 and 192.168.123.0/24 networks
do not belong to the same major network. Likewise, the 66.115.33.0/24
network will be summarized to 66.0.0.0/8.
An auto-summary route will be advertised as a normal internal EIGRP
route. The best (lowest) metric from among the summarized routes will be
applied to this summary route.
The router that performed the auto-summarization will also add the
summary route to its routing table, with a next hop of the Null0 interface.
This is to prevent routing loops.
This auto-summarization can be disabled:
RouterA(config)# router eigrp 10
RouterA(config-router)# no auto-summary
The no auto-summary command will prevent Router A from summarizing
the 10.1.0.0/16 and 66.115.33.0/24 networks.
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#summarization)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
102 EIGRP Manual Summarization
In some instances, it is necessary to manually summarize networks.
For example, you may not want certain networks to be auto-summarized, but
other specific networks should be summarized. In this instance,
summarization can be manually applied using the following interface
configuration command: RouterA(config)# int s0
RouterA(config-if)# ip summary-address eigrp 10 66.0.0.0 255.0.0.0
Recall that auto-summarization had been previously disabled on Router A to
allow the 10.1.0.0/16 network to be advertised correctly. However, this
would also mean that the 66.115.33.0/24 network would not be summarized
as well.
The ip summary-address command allows us to manually summarize this
network. Notice that we configure this on the interface that will be
advertising this network to the other routers.
The manually-created summary route is not advertised as an internal EIGRP
route, but instead is classified as an EIGRP summary route. An EIGRP
summary route has an Administrative Distance of 5, as opposed to an AD of
90 for internal routes.
As with auto-summarization, the router performing manual summarization
will add the summary route to its routing table, with a next hop of the Null0
interface.
The summary route will only stay in the routing table if a more specific
route still exists.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
103 EIGRP Authentication
EIGRP supports authentication to secure routing updates.
The first step is creating a shared authentication key that must be identical on
both routers. This is accomplished in global configuration mode:
RouterA(config)# key chain MYCHAIN
RouterA(config-keychain)# key 1 RouterA(config-keychain-key)# key-string MYPASSWORD
RouterB(config)# key chain MYCHAIN
RouterB(config-keychain)# key 1 RouterB(config-keychain-key)# key-string MYPASSWORD
The first command creates a key chain called MYCHAIN. We must then
associate a key to our keychain. Then we actually configure the shared key
using the key-string command.
We then apply our key chain to the interface connecting to the other router:
RouterA(config)# interface s0
RouterA(config-if)# ip authentication key-chain eigrp 10 MYCHAIN
RouterB(config)# interface s0
RouterB(config-if)# ip authentication key-chain eigrp 10 MYCHAIN
If there was another router off of Router B’s Ethernet port, we could create a
separate key chain with a different key-string. Every router on the EIGRP
domain does not need to use the same key chain, only interfaces directly
connecting two (or more) routers.
The final step in configuring authentication is identifying which encryption
to use. Unlike RIP, EIGRP only supports MD5 encryption:
RouterA(config)# interface s0
RouterA(config-if)# ip authentication mode eigrp 10 md5
Please note that configuring authentication for EIGRP is similar to that of
RIP, but there are slight variations in the commands, including the addition
of the EIGRP Autonomous System Number.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
104 EIGRP Load-Balancing
By default, EIGRP will automatically load-balance across equal-metric
routes (four by default, six maximum). EIGRP also supports load-balancing
across routes with an unequal metric.
Consider the following example:
Earlier in this section, we established that Router A would choose the route
through Router D as its Feasible Distance to the destination network. The
route through Router B became a Feasible Successor.
By default, EIGRP will not load-balance between these two routes, as their
metrics are different (11 through Router D, 16 through Router B). We must
use the variance command to tell EIGRP to load-balance across these
unequal-metric links:
RouterA(config)# router eigrp 10
RouterA(config-router)# variance 2 RouterA(config-router)# maximum-paths 6
The variance command assigns a “multiplier,” in this instance of 2. We
multiply this variance value by the metric of our Feasible Distance (2 x 11
= 22). Thus, any Feasible Successors with a metric within twice that of our
Feasible Distance (i.e. 12 through 22) will now be used for load balancing
by EIGRP.
Remember, only Feasible Successors can be used for load balancing, not
Possibilities (such as the route through Router C).
The maximum-paths command adjusts the number of links EIGRP can loadbalance
across. (Reference: http://www.cisco.com/warp/public/103/19.html)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
105 EIGRP Stubs Router B Router C 10.1.5.1/16 10.2.5.1/16 e0 e0
Router D Router A
10.3.5.1/16 172.16.1.1/16 e0 s0 s0 172.19.1.1/16 172.18.1.1/16 172.16.1.2/16 172.18.1.2/16 172.19.1.2/16 s2 s1 s0
Consider the above hub-and-spoke environment. If Router C were to fail,
Router A (the hub router) would mark the 10.2.0.0/16 route as Active, and
send out Query packets to the spoke routers for an alternate path.
However, it is obvious that no other route exists to the 10.2.0.0/16 network.
Thus, the querying process is a waste of bandwidth and resources.
To prevent unnecessary querying, “spoke” routers in a hub-and-spoke
environment can be configured as Stub routers. A stub router builds a
neighbor adjacency with its hub router(s), and will inform neighbors of its
stub status. The stub router will still build the full topology table.
However, the stub router will immediately respond to any Query packets
with an Inaccessible message. Neighbors will eventually stop querying the
stub router, which helps EIGRP converge quicker and conserves bandwidth.
Configuration of an EIGRP stub is always performed on the spoke router:
RouterB(config)# router eigrp 10
RouterB(config-router)# eigrp stub connected
The eigrp stub command configures this router as Stub, and supports four
possible parameters:
• Receive-only – router will not share updates with neighbors
• Connected – router will only advertise connected networks
• Static – router will only advertise static networks
• Summary – router will only advertise summary routes
The connected and static parameters will only advertise those networks if
they have been injected into the EIGRP process, either using network
statements or using route redistribution. By default, EIGRP stubs will only
send connected and summary routes to neighbors.
(Reference: http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120limit/120s/120s15/eigrpstb.htm)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
106
EIGRP, Frame-Relay, and Bandwidth
Recall that EIGRP’s default bandwidth on serial links is set to 1.544 Mbps
(specifically, 1544). Bandwidth on LAN interfaces (such as Ethernet) is set
to the actual physical speed of the link. For point-to-point PPP or HDLC
links, bandwidth should be manually adjusted to the line’s physical speed.
Additional considerations exist when using Frame-Relay. Observe the
following diagram. Assume the Detroit router’s connection into the Frame-
Relay cloud is 256 Kbps (shared between the Chicago and Houston PVCs).
Detroit Chicago Houston Frame-Relay Cloud
Cisco specifies three rules regarding EIGRP over Frame-Relay:
• The configured bandwidth (and the percentage of bandwidth EIGRP
can use) for a PVC cannot exceed the bandwidth of the PVC (CIR).
• The bandwidth for EIGRP across all PVCs on an interface cannot
exceed the physical bandwidth of the interface
• The bandwidth for EIGRP must be identical on both ends of a PVC.
Consider if router Detroit was configured using Frame-Relay point-tomultipoint,
using no sub-interfaces. Assume also that no bandwidth
command is configured on the physical interface. EIGRP will assume that
the bandwidth is evenly split between all PVCs. In the above scenario,
EIGRP would assume that each PVC was allocated 128 Kbps.
If the CIR for the PVCs were not equal – say, Detroit to Chicago is 56Kbps,
and Detroit to Houston is 256Kbps – the bandwidth should be calculated by
multiplying the bandwidth of the slowest PVC with the total number of
PVCs. In this scenario, the bandwidth should be set to 128Kbps.
(Reference: http://www.cisco.com/warp/public/103/12.html)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
107
EIGRP, Frame-Relay, and Split-Horizon
Detroit Chicago Houston Frame-Relay Cloud
Observe the above Frame-Relay network. We have two possible
configuration options for Detroit:
• Configure frame-relay map statements on the physical interface
• Create separate subinterfaces for each link, treating them as separate
point-to-points.
If choosing the latter, EIGRP will treat each subinterface as a separate link,
and routing will occur with no issue.
If choosing the former, EIGRP will be faced with a split-horizon issue.
Updates from Houston will not be forwarded to Chicago, and visa versa, as
split horizon prevents an update from being sent out the link it was received
on.
Thus, we must disable split horizon for EIGRP:
Detroit(config)# interface s0/0
Detroit(config-router)# no ip split-horizon eigrp 10
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
108 Troubleshooting EIGRP
To view the EIGRP Neighbor Table:
Router# show ip eigrp neighbor
IP-EIGRP neighbors for process 10
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(sec) (ms) Cnt Num
0 172.16.1.2 S0 13 00:00:53 32 200 0 2
0 172.18.1.2 S2 11 00:00:59 32 200 0 3
To view the EIGRP Topology Table, containing all EIGRP route
information:
Router# show ip eigrp topology
IP-EIGRP Topology Table for AS(10)/ID(172.19.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.3.0.0/16, 1 successors, FD is 2297856
via 172.16.1.2 (2297856/128256), Serial0
P 172.19.0.0/16, 1 successors, FD is 281600
via Connected, Serial 1
P 172.18.0.0/16, 1 successors, FD is 128256
via Connected, Serial 2
P 172.16.0.0/16, 1 successors, FD is 2169856
via Connected, Serial0
To view information on EIGRP traffic sent and received on a router:
Router# show ip eigrp traffic
IP-EIGRP Traffic Statistics for process 10
Hellos sent/received: 685/429 Updates sent/received: 4/3 Queries sent/received: 0/0 Replies sent/received: 0/0 Acks sent/received: 1/2
Input queue high water mark 1, 0 drops
SIA-Queries sent/received: 0/0 SIA-Replies sent/received: 0/0
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
109 Troubleshooting EIGRP (continued)
To view the bandwidth, delay, load, reliability and MTU values of an
interface:
Router# show interface s0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 172.16.1.1/16
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
<irrelevant output removed>
To view information specific to the EIGRP protocol:
Router# show ip protocols
Routing Protocol is "eigrp 10"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 10
Automatic network summarization is not in effect
Maximum path: 4 Routing for Networks: 172.16.0.0 172.18.0.0 172.19.0.0 Routing Information Sources:
Gateway Distance Last Update
(this router) 90 00:26:11
172.16.1.2 90 00:23:49
Distance: internal 90 external 170
This command provides us with information on EIGRP timers, EIGRP
metrics, summarization, and the specific networks RIP is advertising.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
110 Troubleshooting EIGRP (continued)
To view the IP routing table:
Router# show ip route
Gateway of last resort is not set
C 172.16.0.0 is directly connected, Serial0
C 172.19.0.0 is directly connected, Serial1
D 10.3.0.0 [90/2297856] via 172.16.1.2, 00:00:15, Serial0
To view a specific route within the IP routing table:
Router# show ip route 10.3.0.0
Routing entry for 10.3.0.0/16
Known via “eigrp 10”, distance 90, metric 2297856 type internal
Last update from 172.16.1.2 on Serial 0, 00:00:15 ago
To debug EIGRP in realtime:
Router# debug eigrp neighbors
Router# debug eigrp packet
Router# debug eigrp route
Router# debug eigrp summary
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
111 Section 12
- Open Shortest Path First -
OSPF (Open Shortest Path First)
OSPF is a standardized Link-State routing protocol, designed to scale
efficiently to support larger networks.
OSPF adheres to the following Link State characteristics:
• OSPF employs a hierarchical network design using Areas.
• OSPF will form neighbor relationships with adjacent routers in the
same Area.
• Instead of advertising the distance to connected networks, OSPF
advertises the status of directly connected links using Link-State
Advertisements (LSAs).
• OSPF sends updates (LSAs) when there is a change to one of its links,
and will only send the change in the update. LSAs are additionally
refreshed every 30 minutes.
• OSPF traffic is multicast either to address 224.0.0.5 (all OSPF
routers) or 224.0.0.6 (all Designated Routers).
• OSPF uses the Dijkstra Shortest Path First algorithm to determine
the shortest path.
• OSPF is a classless protocol, and thus supports VLSMs.
Other characteristics of OSPF include:
• OSPF supports only IP routing.
• OSPF routes have an administrative distance is 110.
• OSPF uses cost as its metric, which is computed based on the
bandwidth of the link. OSPF has no hop-count limit.
The OSPF process builds and maintains three separate tables:
• A neighbor table – contains a list of all neighboring routers.
• A topology table – contains a list of all possible routes to all known
networks within an area.
• A routing table – contains the best route for each known network.
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112 OSPF Neighbors
OSPF forms neighbor relationships, called adjacencies, with other routers in
the same Area by exchanging Hello packets to multicast address 224.0.0.5.
Only after an adjacency is formed can routers share routing information.
Each OSPF router is identified by a unique Router ID. The Router ID can
be determined in one of three ways:
• The Router ID can be manually specified.
• If not manually specified, the highest IP address configured on any
Loopback interface on the router will become the Router ID.
• If no loopback interface exists, the highest IP address configured on
any Physical interface will become the Router ID.
By default, Hello packets are sent out OSPF-enabled interfaces every 10
seconds for broadcast and point-to-point interfaces, and 30 seconds for nonbroadcast
and point-to-multipoint interfaces.
OSPF also has a Dead Interval, which indicates how long a router will wait
without hearing any hellos before announcing a neighbor as “down.” Default
for the Dead Interval is 40 seconds for broadcast and point-to-point
interfaces, and 120 seconds for non-broadcast and point-to-multipoint
interfaces. Notice that, by default, the dead interval timer is four times the
Hello interval.
These timers can be adjusted on a per interface basis:
Router(config-if)# ip ospf hello-interval 15
Router(config-if)# ip ospf dead-interval 60
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
113 OSPF Neighbors (continued)
OSPF routers will only become neighbors if the following parameters within
a Hello packet are identical on each router:
• Area ID
• Area Type (stub, NSSA, etc.)
• Prefix • Subnet Mask • Hello Interval • Dead Interval
• Network Type (broadcast, point-to-point, etc.)
• Authentication
The Hello packets also serve as keepalives to allow routers to quickly
discover if a neighbor is down. Hello packets also contain a neighbor field
that lists the Router IDs of all neighbors the router is connected to.
A neighbor table is constructed from the OSPF Hello packets, which
includes the following information:
• The Router ID of each neighboring router
• The current “state” of each neighboring router
• The interface directly connecting to each neighbor
• The IP address of the remote interface of each neighbor
(Reference: http://www.cisco.com/warp/public/104/29.html)
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
114 OSPF Designated Routers
In multi-access networks such as
Ethernet, there is the possibility of
many neighbor relationships on the
same physical segment. In the above
example, four routers are connected
into the same multi-access segment.
Using the following formula (where
“n” is the number of routers):
n(n-1)/2
…..it is apparent that 6 separate adjacencies are needed for a fully meshed
network. Increase the number of routers to five, and 10 separate adjacencies
would be required. This leads to a considerable amount of unnecessary Link
State Advertisement (LSA) traffic.
If a link off of Router A were to fail, it would flood this information to all
neighbors. Each neighbor, in turn, would then flood that same information to
all other neighbors. This is a waste of bandwidth and processor load.
To prevent this, OSPF will elect a Designated Router (DR) for each multiaccess
networks, accessed via multicast address 224.0.0.6. For redundancy
purposes, a Backup Designated Router (BDR) is also elected.
OSPF routers will form adjacencies with the DR and BDR. If a change
occurs to a link, the update is forwarded only to the DR, which then
forwards it to all other routers. This greatly reduces the flooding of LSAs.
DR and BDR elections are determined by a router’s OSPF priority, which
is configured on a per-interface basis (a router can have interfaces in
multiple multi-access networks). The router with the highest priority
becomes the DR; second highest becomes the BDR. If there is a tie in
priority, whichever router has the highest Router ID will become the DR.
To change the priority on an interface:
Router(config-if)# ip ospf priority 125
Default priority on Cisco routers is 1. A priority of 0 will prevent the router
from being elected DR or BDR. Note: The DR election process is not
preemptive. Thus, if a router with a higher priority is added to the network, it
will not automatically supplant an existing DR. Thus, a router that should
never become the DR should always have its priority set to 0.
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115 OSPF Neighbor States
Neighbor adjacencies will progress through several states, including:
Down – indicates that no Hellos have been heard from the neighboring
router.
Init – indicates a Hello packet has been heard from the neighbor, but twoway
communication has not yet been initialized.
2-Way – indicates that bidirectional communication has been established.
Recall that Hello packets contain a neighbor field. Thus, communication is
considered 2-Way once a router sees its own Router ID in its neighbor’s
Hello Packet. Designated and Backup Designated Routers are elected at
this stage.
ExStart – indicates that the routers are preparing to share link state
information. Master/slave relationships are formed between routers to
determine who will begin the exchange.
Exchange – indicates that the routers are exchanging Database Descriptors
(DBDs). DBDs contain a description of the router’s Topology Database. A
router will examine a neighbor’s DBD to determine if it has information to
share.
Loading – indicates the routers are finally exchanging Link State
Advertisements, containing information about all links connected to each
router. Essentially, routers are sharing their topology tables with each other.
Full – indicates that the routers are fully synchronized. The topology table of
all routers in the area should now be identical. Depending on the “role” of
the neighbor, the state may appear as:
• Full/DR – indicating that the neighbor is a Designated Router (DR)
• Full/BDR – indicating that the neighbor is a Backup Designated
Router (BDR)
• Full/DROther – indicating that the neighbor is neither the DR or
BDR
On a multi-access network, OSPF routers will only form Full adjacencies
with DRs and BDRs. Non-DRs and non-BDRs will still form adjacencies,
but will remain in a 2-Way State. This is normal OSPF behavior.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
116 OSPF Network Types
OSPF’s functionality is different across several different network topology
types. OSPF’s interaction with Frame Relay will be explained in another
section
Broadcast Multi-Access – indicates a topology where broadcast occurs.
• Examples include Ethernet, Token Ring, and ATM.
• OSPF will elect DRs and BDRs.
• Traffic to DRs and BDRs is multicast to 224.0.0.6. Traffic from
DRs and BDRs to other routers is multicast to 224.0.0.5.
• Neighbors do not need to be manually specified.
Point-to-Point – indicates a topology where two routers are directly
connected.
• An example would be a point-to-point T1.
• OSPF will not elect DRs and BDRs.
• All OSPF traffic is multicast to 224.0.0.5.
• Neighbors do not need to be manually specified.
Point-to-Multipoint – indicates a topology where one interface can connect
to multiple destinations. Each connection between a source and destination
is treated as a point-to-point link.
• An example would be Point-to-Multipoint Frame Relay.
• OSPF will not elect DRs and BDRs.
• All OSPF traffic is multicast to 224.0.0.5.
• Neighbors do not need to be manually specified.
Non-broadcast Multi-access Network (NBMA) – indicates a topology
where one interface can connect to multiple destinations; however,
broadcasts cannot be sent across a NBMA network.
• An example would be Frame Relay.
• OSPF will elect DRs and BDRs.
• OSPF neighbors must be manually defined, thus All OSPF traffic
is unicast instead of multicast.
Remember: on non-broadcast networks, neighbors must be manually
specified, as multicast Hello’s are not allowed.
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117
Configuring OSPF Network Types
The default OSPF network type for basic Frame Relay is Non-broadcast
Multi-access Network (NBMA). To configure manually:
Router(config)# interface s0 Router(config-if)# encapsulation frame-relay
Router(config-if)# frame-relay map ip 10.1.1.1 101
Router(config-if)# ip ospf network non-broadcast
Router(config)# router ospf 1
Router(config-router)# neighbor 10.1.1.1
Notice that the neighbor was manually specified, as multicasting is not
allowed on an NBMA. However, the Frame-Relay network can be tricked
into allowing broadcasts, eliminating the need to manually specify
neighbors: Router(config)# interface s0 Router(config-if)# encapsulation frame-relay
Router(config-if)# frame-relay map ip 10.1.1.1 101 broadcast
Router(config-if)# ip ospf network broadcast
Notice that the ospf network type has been changed to broadcast, and the
broadcast parameter was added to the frame-relay map command. The
neighbor no longer needs to be specified, as multicasts will be allowed out
this map.
The default OSPF network type for Ethernet and Token Ring is Broadcast
Multi-Access. To configure manually:
Router(config)# interface e0
Router(config-if)# ip ospf network broadcast
The default OSPF network type for T1’s (HDLC or PPP) and Point-to-Point
Frame Relay is Point-to-Point. To configure manually:
Router(config)# interface s0 Router(config-if)# encapsulation frame-relay
Router(config)# interface s0.1 point-to-point
Router(config-if)# frame-relay map ip 10.1.1.1 101 broadcast
Router(config-if)# ip ospf network point-to-point
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118
Configuring OSPF Network Types (continued)
The default OSPF network type for Point-to-Multipoint Frame Relay is still
Non-broadcast Multi-access Network (NBMA). However, OSPF supports
an additional network type called Point-to-Multipoint, which will allow
neighbor discovery to occur automatically. To configure:
Router(config)# interface s0 Router(config-if)# encapsulation frame-relay
Router(config)# interface s0.2 multipoint
Router(config-if)# frame-relay map ip 10.1.1.1 101 broadcast
Router(config-if)# ip ospf network point-to-multipoint
Additionally, a non-broadcast parameter can be added to the ip ospf network
command when specifying point-to-multipoint.
Router(config)# interface s0 Router(config-if)# encapsulation frame-relay
Router(config)# interface s0.2 multipoint
Router(config-if)# frame-relay map ip 10.1.1.1 101
Router(config-if)# ip ospf network point-to-multipoint non-broadcast
Router(config)# router ospf 1
Router(config-router)# neighbor 10.1.1.1
Notice the different in configuration. The frame-relay map command no
longer has the broadcast parameter, as broadcasts and multicasts are not
allowed on a non-broadcast network.
Thus, in the OSPF router configuration, neighbors must again be manually
specified. Traffic to those neighbors will be unicast instead of multicast.
OSPF network types must be set identically on two “neighboring” routers,
otherwise they will never form an adjacency.
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
119 The OSPF Hierarchy
OSPF is a hierarchical system that separates an Autonomous System into
individual areas. OSPF traffic can either be intra-area (within one area),
inter-area (between separate areas), or external (from another AS).
OSPF routers build a Topology Database of all links within their area, and
all routers within an area will have an identical topology database. Routing
updates between these routers will only contain information about links local
to their area. Limiting the topology database to include only the local area
conserves bandwidth and reduces CPU loads.
Area 0 is required for OSPF to function, and is considered the “Backbone”
area. As a rule, all other areas must have a connection into Area 0, though
this rule can be bypassed using virtual links (explained shortly). Area 0 is
often referred to as the transit area to connect all other areas.
OSPF routers can belong to multiple areas, and will thus contain separate
Topology databases for each area. These routers are known as Area Border
Routers (ABRs).
Consider the above example. Three areas exist: Area 0, Area 1, and Area 2.
Area 0, again, is the backbone area for this Autonomous System. Both Area
1 and Area 2 must directly connect to Area 0.
Routers A and B belong fully to Area 1, while Routers E and F belong fully
to Area 2. These are known as Internal Routers.
Router C belongs to both Area 0 and Area 1. Thus, it is an ABR. Because it
has an interface in Area 0, it can also be considered a Backbone Router.
The same can be said for Router D, as it belongs to both Area 0 and Area 2.
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120
The OSPF Hierarchy (continued)
Now consider the above example. Router G has been added, which belongs
to Area 0. However, Router G also has a connection to the Internet, which is
outside this Autonomous System.
This makes Router G an Autonomous System Border Router (ASBR). A
router can become an ASBR in one of two ways:
• By connecting to a separate Autonomous System, such as the Internet
• By redistributing another routing protocol into the OSPF process.
ASBRs provide access to external networks. OSPF defines two “types” of
external routes:
• Type 2 (E2) – Includes only the external cost to the destination
network. External cost is the metric being advertised from outside the
OSPF domain. This is the default type assigned to external routes.
• Type 1 (E1) – Includes both the external cost, and the internal cost to
reach the ASBR, to determine the total metric to reach the destination
network. Type 1 routes are always preferred over Type 2 routes to the
same destination.
Thus, the four separate OSPF router types are as follows:
• Internal Routers – all router interfaces belong to only one Area.
• Area Border Routers (ABRs) – contains interfaces in at least two
separate areas
• Backbone Routers – contain at least one interface in Area 0
• Autonomous System Border Routers (ASBRs) – contain a
connection to a separate Autonomous System
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121
LSAs and the OSPF Topology Database
OSPF, as a link-state routing protocol, does not rely on routing-by-rumor as
RIP and IGRP do.
Instead, OSPF routers keep track of the status of links within their respective
areas. A link is simply a router interface. From these lists of links and their
respective statuses, the topology database is created. OSPF routers forward
link-state advertisements (LSAs) to ensure the topology database is
consistent on each router within an area.
Several LSA types exist:
• Router LSA (Type 1) – Contains a list of all links local to the router, and
the status and “cost” of those links. Type 1 LSAs are generated by all
routers in OSPF, and are flooded to all other routers within the local area.
• Network LSA (Type 2) – Generated by all Designated Routers in OSPF,
and contains a list of all routers attached to the Designated Router.
• Network Summary LSA (Type 3) – Generated by all ABRs in OSPF,
and contains a list of all destination networks within an area. Type 3
LSAs are sent between areas to allow inter-area communication to occur.
• ASBR Summary LSA (Type 4) – Generated by ABRs in OSPF, and
contains a route to any ASBRs in the OSPF system. Type 4 LSAs are
sent from an ABR into its local area, so that Internal routers know how to
exit the Autonomous System.
• External LSA (Type 5) – Generated by ASBRs in OSPF, and contain
routes to destination networks outside the local Autonomous System.
Type 5 LSAs can also take the form of a default route to all networks
outside the local AS. Type 5 LSAs are flooded to all areas in the OSPF
system.
Multicast OSPF (MOSPF) utilizes a Type 6 LSA, but that goes beyond the
scope of this guide.
Later in this section, Type 7 NSSA External LSAs will be described in
detail.
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122
LSAs and the OSPF Topology Database (continued)
From the above example, the following can be determined:
• Routers A, B, E, and F are Internal Routers.
• Routers C and D are ABRs.
• Router G is an ASBR.
All routers will generate Router (Type 1) LSAs. For example, Router A
will generate a Type 1 LSA that contains the status of links FastEthernet 0/0
and FastEthernet 0/1. This LSA will be flooded to all other routers in Area 1.
Designated Routers will generate Network (Type 2) LSAs. For example, if
Router C was elected the DR for the multi-access network in Area 1, it
would generate a Type 2 LSA containing a list of all routers attached to it.
Area Border Routers (ABRs) will generate Network Summary (Type 3)
LSAs. For example, Router C is an ABR between Area 0 and Area 1. It will
thus send Type 3 LSAs into both areas. Type 3 LSAs sent into Area 0 will
contain a list of networks within Area 1, including costs to reach those
networks. Type 3 LSAs sent into Area 1 will contain a list of networks
within Area 0, and all other areas connected to Area 0. This allows Area 1 to
reach any other area, and all other areas to reach Area 1.
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123
LSAs and the OSPF Topology Database (continued)
ABRs will also generate ASBR Summary (Type 4) LSAs. For example,
Router C will send Type 4 LSAs into Area 1 containing a route to the
ASBR, thus providing routers in Area 1 with the path out of the
Autonomous System.
ASBRs will generate External (Type 5) LSAs. For example, Router G will
generate Type 5 LSAs that contain routes to network outside the AS. These
Type 5 LSAs will be flooded to routers of all areas.
Each type of LSA is propagated under three circumstances:
• When a new adjacency is formed.
• When a change occurs to the topology table.
• When an LSA reaches its maximum age (every 30 minutes, by
default).
Thus, though OSPF is typically recognized to only send updates when a
change occurs, LSA’s are still periodically refreshed every 30 minutes.
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124 The OSPF Metric
OSPF determines the best (or shortest) path to a destination network using a
cost metric, which is based on the bandwidth of interfaces. The total cost of
a route is the sum of all outgoing interface costs. Lowest cost is preferred.
Cisco applies default costs to specific interface types:
Type Cost Serial (56K) 1785 Serial (64K) 1562 T1 (1.544Mbps) 64
Token Ring (4Mbps) 25
Ethernet (10 Mbps) 10
Token Ring (16 Mbps) 6
Fast Ethernet 1
On Serial interfaces, OSPF will use the configured bandwidth (measured in
Kbps) to determine the cost:
Router(config)# interface s0 Router(config-if)# bandwidth 64
The default cost of an interface can be superseded:
Router(config)# interface e0
Router(config-if)# ip ospf cost 5
Changing the cost of an interface can alter which path OSPF deems the
“shortest,” and thus should be used with great care.
To alter how OSPF calculates its default metrics for interfaces:
Router(config)# router ospf 1
Router(config-router)# ospf auto-cost reference-bandwidth 100
The above ospf auto-cost command has a value of 100 configured, which is
actually the default. This indicates that a 100Mbps link will have a cost of 1
(because 100/100 is 1). All other costs are based off of this. For example, the
cost of 4 Mbps Token Ring is 25 because 100/4 = 25.
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125 Configuring Basic OSPF
Routing protocol configuration occurs in Global Configuration mode. On
Router A, to configure OSPF:
RouterA(config)# router ospf 1
RouterA(config-router)# router-id 1.1.1.1
RouterA(config-router)# network 172.16.0.0 0.0.255.255 area 1
RouterA(config-router)# network 172.17.0.0 0.0.255.255 area 0
The first command, router ospf 1, enables the OSPF process. The “1”
indicates the OSPF process ID, and can be unique on each router. The
process ID allows multiple OSPF processes to run on the same router. The
router-id command assigns a unique OSPF ID of 1.1.1.1 for this router.
Note the use of a wildcard mask instead of a subnet mask in the network
statement. With OSPF, we’re not telling the router what networks to
advertise; we’re telling the router to place certain interfaces into specific
areas, so those routers can form neighbor relationships. The wildcard mask
0.0.255.255 tells us that the last two octets can match any number.
The first network statement places interface E0 on Router A into Area 1.
Likewise, the second network statement places interface S0 on Router A into
Area 0. The network statement could have been written more specifically:
RouterA(config)# router ospf 1
RouterA(config-router)# network 172.16.1.2 0.0.0.0 area 1
RouterA(config-router)# network 172.17.1.1 0.0.0.0 area 0
In order for Router B to form a neighbor relationship with Router A, its
connecting interface must be put in the same Area as Router A:
RouterB(config)# router ospf 1
RouterA(config-router)# router-id 2.2.2.2
RouterB(config-router)# network 172.17.1.2 0.0.0.0 area 0
RouterB(config-router)# network 172.18.1.1 0.0.0.0 area 2
If Router B’s S0 interface was placed in a different area than Router A’s S0
interface, the two routers would never form a neighbor relationship, and
never share routing updates.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
126 OSPF Passive-Interfaces
It is possible to control which router interfaces will participate in the OSPF
process. Just as with EIGRP and RIP, we can use the passive-interface
command.
However, please note that the passive-interface command works differently
with OSPF than with RIP or IGRP. OSPF will no longer form neighbor
relationships out of a “passive” interface, thus this command prevents
updates from being sent or received out of this interface:
RouterC(config)# router ospf 1
RouterC(config-router)# network 10.4.0.0 0.0.255.255 area 0
RouterC(config-router)# network 10.2.0.0 0.0.255.255 area 0
RouterC(config-router)# passive-interface s0
Router C will not form a neighbor adjacency with Router B.
It is possible to configure all interfaces to be passive using the passiveinterface
default command, and then individually use the no passiveinterface
command on the interfaces that neighbors should be formed on:
RouterC(config)# router ospf 1
RouterC(config-router)# network 10.4.0.0 0.0.255.255 area 0
RouterC(config-router)# network 10.2.0.0 0.0.255.255 area 0
RouterC(config-router)# passive-interface default
RouterC(config-router)# no passive-interface e0
Always remember, that the passive-interface command will prevent OSPF
(and EIGRP) from forming neighbor relationships out of that interface. No
routing updates are passed in either direction.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
127 OSPF Authentication
OSPF supports authentication to secure routing updates. However, OSPF
authentication is configured differently than RIP or EIGRP authentication.
Two forms of OSPF authentication exist, using either clear-text or an MD5
hash. To configure clear-text authentication, the first step is to enable
authentication for the area, under the OSPF routing process:
RouterA(config)# router ospf 1
RouterA(config-router)# network 172.17.0.0 0.0.255.255 area 0
RouterA(config-router)# area 0 authentication
Then, the authentication key must be configured on the interface:
RouterA(config)# interface s0
RouterA(config-if)# ip ospf authentication
RouterA(config-if)# ip ospf authentication-key MYKEY
To configure MD5-hashed authentication, the first step is also to enable
authentication for the area under the OSPF process:
RouterA(config)# router ospf 1
RouterA(config-router)# network 172.17.0.0 0.0.255.255 area 0
RouterA(config-router)# area 0 authentication message-digest
Notice the additional parameter message-digest included with the area 0
authentication command. Next, the hashed authentication key must be
configured on the interface:
RouterA(config)# interface s0
RouterA(config-router)# ip ospf message-digest-key 10 md5 MYKEY
Area authentication must be enabled on all routers in the area, and the form
of authentication must be identical (clear-text or MD5). The authentication
keys do not need to be the same on every router in the OSPF area, but must
be the same on interfaces connecting two neighbors.
Please note: if authentication is enabled for Area 0, the same authentication
must be configured on Virtual Links, as they are “extensions” of Area 0.
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128 OSPF Virtual Links
Earlier in this guide, it was stated that all areas must directly connect into
Area 0, as a rule. In the above example, Area 2 has no direct connection to
Area 0, but must transit through Area 1 to reach the backbone area. In
normal OSPF operation, this shouldn’t be possible.
There may be certain circumstances that may prevent an area from directly
connecting into Area 0. Virtual links can be used as a workaround, to
logically connect separated areas to Area 0. In the above example, a virtual
link would essentially create a tunnel from Area 2 to Area 0, using Area 1 a
transit area. One end of the Virtual Link must be connected to Area 0.
Configuration occurs on the Area Border Routers (ABRs) connecting Area
1 to Area 2 (Router B), and Area 1 to Area 0 (Router C). Configuration on
Router B would be as follows:
RouterB(config)# router ospf 1
RouterB(config-router)# router-id 2.2.2.2
RouterB(config-router)# area 1 virtual-link 3.3.3.3
The first command enables the ospf process. The second command manually
sets the router-id for Router B to 2.2.2.2.
The third command actually creates the virtual-link. Notice that it specifies
area 1, which is the transit area. Finally, the command points to the remote
ABR’s Router ID of 3.3.3.3.
Configuration on Router C would be as follows:
RouterC(config)# router ospf 1
RouterC(config-router)# router-id 3.3.3.3
RouterC(config-router)# area 1 virtual-link 2.2.2.2
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129
OSPF Virtual Links (continued)
It is also possible to have two separated (or discontiguous) Area 0’s. In order
for OSPF to function properly, the two Area 0’s must be connected using a
virtual link.
Again, configuration occurs on the transit area’s ABRs:
RouterB(config)# router ospf 1
RouterB(config-router)# router-id 2.2.2.2
RouterB(config-router)# area 1 virtual-link 3.3.3.3
RouterC(config)# router ospf 1
RouterC(config-router)# router-id 3.3.3.3
RouterC(config-router)# area 1 virtual-link 2.2.2.2
Always remember: the area specified in the virtual-link command is the
transit area. Additionally, the transit area cannot be a stub area.
As stated earlier, if authentication is enabled for Area 0, the same
authentication must be configured on Virtual Links, as they are “extensions”
of Area 0:
RouterB(config)# router ospf 1
RouterB(config-router)# area 1 virtual-link 3.3.3.3 message-digest-key 1 md5 MYKEY
RouterC(config)# router ospf 1
RouterC(config-router)# area 1 virtual-link 2.2.2.2 message-digest-key 1 md5 MYKEY
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
130 Inter-Area OSPF Summarization
Consider the above example. OSPF is a classless routing protocol, thus all of
the listed networks would be advertised individually. This increases the size
of the topology databases and routing tables on routers in the domain, and
may be undesirable. Advertising only a summary route for inter-area
communication can reduce the load on router CPUs.
For example, all of the networks in Area 1 can be summarized as
10.1.0.0/21. Similarly, all of the networks in Area 2 can be summarized as
10.1.8.0/21.
Inter-area summarization is configured on Area Border Routers (ABRs).
Configuration on Router A would be as follows:
RouterA(config)# router ospf 1
RouterA(config-router)# network 10.1.0.0 0.0.7.255 area 1
RouterA(config-router)# area 1 range 10.1.0.0 255.255.248.0
The network statement includes all of the 10.1.x.0 networks into Area 1. The
area 1 range command creates a summary route for those networks, which
is then advertised into Area 0, as opposed to each route individually.
Proper design dictates that a static route be created for the summarized
network, pointing to the Null interface. This sends any traffic destined
specifically to the summarized address to the bit-bucket in the sky, in order
to prevent routing loops:
RouterA(config)# ip route 10.1.0.0 255.255.248.0 null0
In IOS versions 12.1(6) and later, this static route is created automatically.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
131 External OSPF Summarization
Consider the above example. Router B is an Autonomous System Border
Router (ASBR). It is possible to redistribute the four “external” networks
into the OSPF system. However, a separate route for each network will be
advertised.
Again, this is wasteful. The four external networks can be summarized as
15.0.0.0/14.
External Summarization is configured on ASBRs, and will only
summarize external routes learned by route redistribution. Configuration on
Router B would be as follows:
RouterB(config)# router ospf 1
RouterB(config-router)# summary-address 15.0.0.0 255.252.0.0
This summarized route is now propagated to all routers in every OSPF area.
Summarization can be used to filter certain routes (true route filtering is
covered in a separate guide). To force OSPF to advertise the 15.0.0.0 and
15.1.0.0 networks as a summarized route, but not advertise the 15.2.0.0 and
15.3.0.0 prefixes:
RouterB(config)# router ospf 1
RouterB(config-router)# summary-address 15.0.0.0 255.254.0.0
RouterB(config-router)# summary-address 15.2.0.0 255.255.0.0 not-advertise
RouterB(config-router)# summary-address 15.3.0.0 255.255.0.0 not-advertise
The first summary-address command summarizes the 15.0.0.0/16 and
15.1.0.0/16 networks to 15.0.0.0/15, and advertises the summary as normal
in the OSPF domain. The next two summary-address commands specifically
reference the 15.2.0.0/16 and 15.3.0.0/16 networks, with the not-advertise
parameter. As implied, these networks will not be advertised in OSPF.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
132 OSPF Area Types
In order to control the propagation of LSAs in the OSPF domain, several
area types were developed.
Standard Area – A “normal” OSPF area.
• Routers within a standard area will share Router (Type 1) and
Network (Type 2) LSAs to build their topology tables. Once fully
synchronized, routers within an area will all have identical
topology tables.
• Standard areas will accept Network Summary (Type 3) LSAs,
which contain the routes to reach networks in all other areas.
• Standard areas will accept ASBR Summary (Type 4) and External
(Type 5) LSAs, which contain the route to the ASBR and routes to
external networks, respectively.
Configuration of standard areas is straight forward:
Router(config)# router ospf 1
Router(config-router)# network 10.1.0.0 0.0.7.255 area 1
Stub Area – Prevents external routes from flooding into an area.
• Like Standard areas, Stub area routers will share Type 1 and Type
2 LSAs to build their topology tables.
• Stub areas will also accept Type 3 LSAs to reach other areas.
• Stub areas will not accept Type 4 or Type 5 LSAs, detailing routes
to external networks.
The purpose of Stub areas is to limit the number of LSAs flooded into the
area, to conserve bandwidth and router CPUs. The Stub’s ABR will
automatically inject a default route into the Stub area, so that those routers
can reach the external networks. The ABR will be the next-hop for the
default route.
Configuration of stub areas is relatively simple:
Router(config)# router ospf 1
Router(config-router)# network 10.1.0.0 0.0.7.255 area 1
Router(config-router)# area 1 stub
The area 1 stub command must be configured on all routers in the Stub area.
No ASBRs are allowed in a Stub area.
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133
OSPF Area Types (continued)
Totally Stubby Area – Prevents both inter-area and external routes from
flooding into an area.
• Like Standard and Stub areas, Totally Stubby area routers will
share Type 1 and Type 2 LSAs to build their topology tables.
• Totally Stubby areas will not accept Type 3 LSAs to other areas.
• Totally Stubby areas will also not accept Type 4 or Type 5 LSAs,
detailing routes to external networks.
Again, the purpose of Totally Stubby areas is to limit the number of LSAs
flooded into the area, to conserve bandwidth and router CPUs. The Stub’s
ABR will instead automatically inject a default route into the Totally
Stubby area, so that those routers can reach both inter-area networks and
external networks. The ABR will be the next-hop for the default route.
Configuration of totally stubby areas is relatively simple:
Router(config)# router ospf 1
Router(config-router)# network 10.1.0.0 0.0.7.255 area 1
Router(config-router)# area 1 stub no-summary
The area 1 stub no-summary command is configured only on the ABR of
the Totally Stubby area; other routers within the area are configured with the
area 1 stub command. No ASBRs are allowed in a Totally Stubby area.
In the above example, if we were to configure Area 1 as a Totally Stubby
area, it would not accept any external routes originating from the ASBR
(Router G). It also would not accept any Type 3 LSAs containing route
information about Area 0 and Area 2. Instead, Router C (the ABR) will
inject a default route into Area 1, and all routers within Area 1 will use
Router C as their gateway to all other networks.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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134
OSPF Area Types (continued)
Not So Stubby Area (NSSA) – Similar to a Stub area; prevents external
routes from flooding into an area, unless those external routes originated
from an ASBR within the NSSA area.
• Like Standard and Stub areas, NSSA area routers will share Type 1
and Type 2 LSAs to build their topology tables.
• NSSA areas will also accept Network Summary (Type 3) LSAs,
which contain the routes to reach networks in all other areas.
• NSSA areas will not accept Type 4 or Type 5 LSAs, detailing
routes to external networks.
• If an ASBR exists within the NSSA area, that ASBR will generate
Type 7 LSAs.
Again, NSSA areas are almost identical to Stub areas. If Area 1 was
configured as an NSSA, it would not accept any external routes originating
from Router G (an ASBR outside Area 1).
However, Area 1 also has an ASBR within the area (Router A). Those
external routes will be flooded into Area 1 as Type 7 LSAs. These external
routes will not be forwarded to other areas as Type 7 LSAs; instead, they
will be converted into Type 5 LSAs by Area 1’s ABR (Router C).
Configuration of NSSA areas is relatively simple:
Router(config)# router ospf 1
Router(config-router)# network 10.1.0.0 0.0.7.255 area 1
Router(config-router)# area 1 nssa
The area 1 nssa command must be applied to all routers in the NSSA area.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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135
OSPF Area Types (continued)
Totally Not So Stubby Area (TNSSA) – Similar to a Totally Stubby area;
prevents both inter-area and external routes from flooding into an area,
unless those external routes originated from an ASBR within the NSSA area.
• Like Standard and Stub areas, TNSSA area routers will share Type
1 and Type 2 LSAs to build their topology tables.
• TNSSA areas will not accept Type 3 LSAs to other areas.
• TNSSA areas will not accept Type 4 or Type 5 LSAs, detailing
routes to external networks.
• If an ASBR exists within the TNSSA area, that ASBR will
generate Type 7 LSAs.
With the exception of not accepting inter-area routes, TNSSA areas are
identical in function to NSSA areas.
Configuration of TNSSA areas is relatively simple:
Router(config)# router ospf 1
Router(config-router)# network 10.1.0.0 0.0.7.255 area 1
Router(config-router)# area 1 nssa no-summary
The area 1 nssa no-summary command is configured only on the ABR of
the TNSSA area; other routers within the area are configured with the area 1
nssa command.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
136
OSPF and Default Routes
We have learned about four types of OSPF areas:
• Standard areas • Stub areas
• Totally Stubby areas
• Not So Stubby areas (NSSA)
The ABRs and ASBRs of Standard areas do not automatically generate (or
inject) default routes into the area. Consider the following example:
Router A Router C
Area 1 Area 0
External Networks Router B
Assume that Area 1 is configured as a Standard area. Router C will forward
Type 3 LSAs from all other areas into Area 1, allowing Router A and Router
B to reach inter-area networks.
Notice also that Router A is an ASBR, connecting to an external
Autonomous System. Thus, Router A will generate Type 5 LSAs, detailing
the routes to these external networks.
To additionally force Router A to generate a default route (indicating itself
as the next hop) for the external networks, and inject this into Area 1. This
default route will be advertised as a Type 5 LSA to all other areas:
RouterA(config)# router ospf 1
RouterA(config-router)# default-information originate
Router A must have a default route in its routing table in order for the above
command to function. Router A’s default route would point to some
upstream router in the external Autonomous System.
If a default route does not exist in its routing table, Router A can still be
forced to advertise a default route using the always parameter:
RouterA(config)# router ospf 1
RouterA(config-router)# default-information originate always
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
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137
OSPF and Default Routes (continued)
The ABRs of Stub and Totally Stubby areas automatically generate (and
inject) a default route (0.0.0.0/0) into the area. Routers in Stub areas use
this default route to reach external networks, while routers in Totally Stubby
areas use the default route to reach both inter-area and external networks.
To control the “cost” metric of the default route in Stub or Totally Stubby
areas (configured on the ABR):
Router(config)# router ospf 1
Router(config-router)# area 1 stub
Router(config-router)# area 1 default-cost 10
The ABRs of NSSA areas must be manually configured to generate (and
inject) a default route into the area:
Router(config)# router ospf 1
Router(config-router)# area 1 nssa default-information-originate
Additionally, the ASBR of an NSSA area can generate and inject a default
route. This default route will be advertised as a Type 7 LSA, as Type 5
LSA’s are not allowed in NSSAs. The command is no different than
injecting a default route from an NSSA ABR:
Router(config)# router ospf 1
Router(config-router)# area 1 nssa default-information-originate
Reference: (http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a74.shtml)
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
138 OSPF SPF Timers
To adjust the SPF timers in OSPF:
Router(config)# router ospf 1
Router(config-router)# timers spf 10 15
The timers spf command includes two parameters, measured in seconds. The
first (10) indicates the SPF-Delay, or how long the OSPF should wait after
receiving a topology change to recalculate the shortest path. The second (15)
indicates the SPF-Holdtime, or how long OSPF should wait in between
separate SPF calculations.
The timers spf command has actually become deprecated. It has been
replaced with:
Router(config)# router ospf 1
Router(config-router)# timers throttle spf 5 10000 80000
The timers throttle spf command includes three parameters, measure in
milliseconds. The first (5) indicates how long OSPF should wait after
receiving a topology change to recalculate the shortest path. The second
(10000) indicates the hold-down time, or how long OSPF should wait in
between separate SPF calculations. If OSPF receives another topology
change during the hold-time interval, it will continue to double the hold-time
interval until it reaches the maximum hold-time (80000).
The purpose of the both SPF timer commands is to prevent OSPF from
constantly converging, if the network links are “flapping.” The timers spf
and timers throttle spf commands cannot be used together.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
139 Advanced OSPF Configuration
To force the OSPF process to ignore OSPF Multicast (Type 6) LSAs:
Router(config)# router ospf 1
Router(config-router)# ignore lsa mospf
To force an interface to filter all outgoing OSPF LSA’s:
Router(config)# interface e0
Router(config-if)# ip ospf database-filter all out
Loopback interfaces are treated differently than other interfaces, when
advertised in OSPF. OSPF will advertise a loopback interface as a specific
“host” route (with a mask of /32 or 255.255.255.255). To force OSPF to
advertise a loopback interface with its proper subnet mask:
Router(config)# interface loopback0
Router(config-if)# ip address 10.50.5.1 255.255.255.0
Router(config-if)# ip ospf network point-to-point
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
140 Troubleshooting OSPF
To view the OSPF Neighbor Table:
Router# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
7.7.7.7 1 FULL/ - 00:00:36 150.50.17.2 Serial0
6.6.6.6 1 FULL/DR 00:00:11 150.50.18.1 Ethernet0
The Neighbor Table provides the following information about each
neighbor:
• The Router ID of the remote neighbor.
• The OSPF priority of the remote neighbor (used for DR/BDR
elections).
• The current neighbor state.
• The dead interval timer.
• The connecting IP address of the remote neighbor.
• The local interface connecting to the remote neighbor.
To view the OSPF topology table:
Router# show ip ospf database
OSPF Router with ID (9.9.9.9) (Process ID 10)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
7.7.7.7 7.7.7.7 329 0x80000007 0x42A0 2
8.8.8.8 8.8.8.8 291 0x80000007 0x9FFC 1
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 7.7.7.7 103 0x80000005 0x13E4
192.168.34.0 7.7.7.7 105 0x80000003 0x345A
The Topology Table provides the following information:
• The actual link (or route).
• The advertising Router ID.
• The link-state age timer.
• The sequence number and checksum for each entry.
(Reference: http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017d02e.html)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
141 Troubleshooting OSPF (continued)
To view the specific information about an OSPF process:
Router# show ip ospf 1
Routing Process "ospf 1" with ID 9.9.9.9
Supports only single TOS(TOS0) routes
Supports opaque LSA
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x0
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 3 times
Area ranges are
Number of LSA 2. Checksum Sum 0xDDEC
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
The show ip ospf command provides the following information:
• The local Router ID.
• SPF Scheduling information, and various SPF timers.
• The number of interfaces in specific areas, including the type of area.
• The link-state age timer.
• The sequence number and checksum for each entry.
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142 Troubleshooting OSPF (continued)
To view OSPF-specific information on an interface:
Router# show ip ospf interface s0
Serial0 is up, line protocol is up
Internet Address 192.168.79.2/24, Area 0
Process ID 10, Router ID 9.9.9.9, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 7.7.7.7
Suppress hello for 0 neighbor(s)
The show ip ospf interface command provides the following information:
• The local Router ID.
• The interface network type.
• The OSPF cost for the interface.
• The interface Hello and Dead timers.
• A list of neighbor adjacencies.
To view routing protocol specific information for OSPF:
Router# show ip protocols
Routing Protocol is “ospf 10"
Invalid after 0 seconds, hold down 0, flushed after 0
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Routing for Networks:
192.168.79.0 0.0.0.255 area 0
192.168.109.0 0.0.0.255 area 0
Routing Information Sources:
Gateway Distance Last Update
7.7.7.7 110 00:01:05
Distance: (default is 110)
The show ip protocols command provides the following information:
• Locally originated networks that are being advertised.
• Neighboring sources for routing information
• The administrative distance of neighboring sources.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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143 Troubleshooting OSPF (continued)
To reset an OSPF process, including neighbor adjacencies:
Router# clear ip ospf process
To display information about OSPF virtual-links:
Router# show ip ospf virtual-links
To display routes to both ABRs and ASBRs:
Router# show ip ospf border-routers
To debug OSPF in realtime:
Router# debug ip ospf adj
Router# debug ip ospf events
Router# debug ip ospf hello
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
144 Section 13 - IS-IS - IS-IS Fundamentals
IS-IS (Intermediate System -to- Intermediate System) is a standardized
link-state protocol that was developed to be the definitive routing protocol
for the OSI (Open Systems Interconnect) Model, which was developed by
ISO (International Standards Organization). IS-IS shares many
similarities to OSPF. Though it was designed as an interior gateway protocol
(IGP), IS-IS is predominantly used by ISPs, due to its scalability.
IS-IS adheres to the following Link State characteristics:
• IS-IS allows for a hierarchical network design using Areas.
• IS-IS will form neighbor relationships with adjacent routers of the
same IS-IS type.
• Instead of advertising the distance to connected networks, IS-IS
advertises the status of directly connected “links” in the form of
Link-State Packets (LSPs). IS-IS will only send out updates when
there is a change to one of its links, and will only send the change in
the update.
• IS-IS uses the Dijkstra Shortest Path First algorithm to determine
the shortest path.
• IS-IS is a classless protocol, and thus supports VLSMs.
Other characteristics of IS-IS include:
• IS-IS was originally developed to route the ISO address space, and
thus is not limited to IP routing.
• IS-IS routes have an administrative distance is 115.
• IS-IS uses an arbitrary cost for its metric. IS-IS additionally has three
optional metrics: delay, expense, and error. Cisco does not support
these optional metrics.
• IS-IS has no hop-count limit.
The IS-IS process builds and maintains three separate tables:
• A neighbor table – contains a list of all neighboring routers.
• A topology table – contains a list of all possible routes to all known
networks within an area.
• A routing table – contains the best route for each known network.
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145
IS-IS Protocols and Addressing
IS-IS consists of three sub-protocols that work in tandem to achieve end-toend
routing which ISO defined as Connectionless Network Service
(CLNS):
• CLNP (Connectionless Network Protocol) – serves as the Layer-3
protocol for IS-IS (and was developed by ISO).
• ES-IS (End System -to- Intermediate System) – used to route
between hosts and routers.
• IS-IS (Intermediate System -to- Intermediate System) – used to
route between routers.
IS-IS was originally developed to route ISO CLNP addresses (outlined in
RFC 1142). However, CLNP addressing never became prominently used.
Thus, IS-IS was modified to additionally support IP routing, and became
Integrated (or Dual) IS-IS (outlined in RFC 1195).
The IS-IS CLNP address is hexadecimal and of variable length, and can
range from 64 to 160 bits in length. The CLNP address contains three
“sections,” including:
• Area field – (variable length)
• ID field – (from 8 to 64 bits, though usually 48 bits)
• Selector (SEL) field - (8 bits)
Thus, the CLNP address identifies the “Area” in which a device is located,
the actual host “ID,” and the destination application on that host, in the form
of the “SEL” field. The CNLP address is logically segmented even further,
as demonstrated by the following table:
IDP DSP
AFI IDI HO-DSP System-ID NSEL
Area Field ID Field SEL Field
Observe the top row of the above figure. The ISO CLNP address provides
granular control by separating internal and external routing information:
• The IDP (Initial Domain Part) portion of the address identifies
the Autonomous System of the device (and is used to route to or
between Autonomous Systems)
• The DSP (Domain Specific Part) portion of the address is used to
route within the autonomous system.
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146
IS-IS Protocols and Addressing (continued)
IDP DSP
AFI IDI HO-DSP System-ID NSEL
Area Field ID Field SEL Field
The IDP portion of the address is separated into two “sections,” including:
• AFI (Authority and Format Identifier) – specifies the
organization authorized to assign addresses, and the format and
length of the rest of the CLNP address. The AFI is always 8 bits.
• IDI (Initial Domain Identifier) – identifies the “suborganization”
under the parent AFI organization. The length of the
IDI is dependent on the chosen AFI.
An AFI of 0x49 indicates a private CLNP address, which cannot be routed
globally (the equivalent of an IPv4 private address). An AFI of 0x47 is
commonly used for global IS-IS networks, with the IDI section identifying
specific organizations.
The AFI plus the IDI essentially identify the autonomous system of the
address. However, this is not the equivalent of a BGP AS number, nor is it
compatible with BGP as an exterior routing protocol.
The DSP portion of the address is separated into three “sections,” including:
• HO-DSP (High Order DSP) – identifies the area within an
autonomous system
• System ID – identifies the specific host. Usually 48 bits (or 6 octets)
in length, to accommodate MAC addresses
• NSEL – identifies the destination upper layer protocol of the host
(always 8 bits)
Two “types” of CLNP addresses are defined:
• NET address – does not contain upper-layer information (in other
words, the SEL field is always set to 0x00)
• NSAP address – the “full” CLNP address, with populated Area, ID,
and SEL fields.
Please note: A NET address is simply an NSAP address with a zero value in
the SEL field.
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147 CLNS Address Example
The following is an example of a full ISO CLNS address:
47.1234.5678.9abc.def0.0001.1111.2222.3333.00
Correlating the above address to the appropriate fields:
IDP DSP
AFI IDI HO-DSP System-ID NSEL
47. 1234.5678.9abc.def0. 0001. 1111.2222.3333. 00
Area Field ID Field SEL Field
The System-ID is usually populated by the device’s MAC address or IP v4
address.
Recall that CLNS addresses are of variable length. We can specify addresses
without an IDI field:
47.0001.1111.2222.3333.00
Thus, the above address contains an AFI (Autonomous System), HO-DSP
(Area), System-ID (in this example, a MAC Address), and the NSEL (SEL).
Because the SEL field has a zero value (0x00), the above address is defined
as a NET address, and not an NSAP address.
ISO CLNS addresses are not applied on an interface-by-interface basis.
Instead, a single CLNS address is applied to the entire device.
Even if Integrated IS-IS is being used (thus indicating that IPv4 is being
routed instead of CLNS), a CLNS address is still required on the IS-IS
router. This is configured under the IS-IS router process.
Routers within the same area must share identical AFI, IDI, and HO-DSP
values, but each must have a unique System-ID
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
148 IS-IS Packet Types
IS-IS defines two categories of network devices:
• ES (End System) – identifies an end host.
• IS (Intermediate System) – identifies a Layer 3 router.
IS-IS additionally defines four categories of packet types:
• Hello • LSP • CSNP • PSNP
Hello packets are exchanged for neighbor discovery. Three types of IS-IS
Hello packets exist:
• IIH (IS-IS Hello) – exchanged between routers (or IS’s) to form
neighbor adjacencies.
• ESH (ES Hello) – sent from an ES to discover a router.
• ISH (IS Hello) – sent from an IS to announce its presence to ES’s
An LSP (Link State Packet) is used to share topology information between
routers. There are separate LSPs for Level 1 and Level 2 routing. LSP’s are
covered in great detail shortly.
A CSNP (Complete Sequence Number PDU) is an update containing the
full link-state database. IS-IS routers will refresh the full database every 15
minutes.
A PSNP (Partial Sequence Number PDU) is used by IS-IS routers to both
request and acknowledge a link-state update.
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149 IS-IS Neighbors
IS-IS routers form neighbor relationships, called adjacencies, by
exchanging Hello packets (often referred to as IS-IS Hellos or IIH’s). Hello
packets are sent out every 10 seconds, regardless of media type. Only after
an adjacency is formed can routers share routing information.
IS-IS supports three IIH packet formats; one for point-to-point links, and
two for broadcast (or LAN) links (Level-1 and Level-2 broadcast Hellos).
Unlike OSPF, IS-IS neighbors do not have to share a common IP subnet to
form an adjacency. Adjacencies are formed across CLNP connections, not
IP connections, even when using Integrated IS-IS. Thus, IS-IS actually
requires no IP connectivity between its routers to route IP traffic!
There are two types of adjacencies:
• Level-1 adjacency – for routing within an area (intra-area routing)
• Level-2 adjacency – for routing between areas (intra-area routing)
IS-IS routers must share a common physical link to become neighbors, and
the System-ID must be unique on each router. Additionally, the following
parameters must be identical on each router:
• Hello packet format (point-to-point or broadcast)
• Hello timers
• Router “level” (explained shortly)
• Area (only for Level-1 adjacencies)
• Authentication parameters (Cisco devices currently support only
clear-text authentication for IS-IS).
• MTU
Neighbors will elect a DIS (Designated Intermediate System) on broadcast
links. A DIS is the equivalent of an OSPF DR (Designated Router). Unlike
OSPF, however, there is no Backup DIS, and thus a new election will occur
immediately if the DIS fails. Additionally, the DIS election is preemptive.
Whichever IS-IS router has the highest priority will be elected the DIS
(default priority is 64). In the event of a tie, whichever IS-IS router has the
highest SNPA (usually MAC) address will become the DIS. The DIS sends
out hello packets every 3.3 seconds, instead of every 10 seconds.
(Reference: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfisis.pdf)
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150 The IS-IS Hierarchy
IS-IS defines three types of IS-IS routers:
• Level-1 Router – contained within a single area, with a topology
table limited to only its local area (called the Level-1 Database)
• Level-2 Router - a backbone router that routes between areas, and
builds a Level-2 Database.
• Level-1-2 Router – similar to an area border router. Interfaces
between a local area and the backbone area, and builds both a Level-1
and a Level-2 database.
Each type of IS-IS router will form only specific adjacencies:
• Level-1 routers form Level-1 adjacencies with other Level-1 routers
and Level-1-2 routers.
• Level-2 routers form Level-2 adjacencies with other Level-2 routers
and Level-1-2 routers.
• Level-1-2 routers form both Level-1 and Level-2 adjacencies with
other Level-1-2 routers.
• Level-1 routers will never form adjacencies with Level-2 routers.
The IS-IS backbone consists of multiple contiguous Level-2 routers, each of
which can exist in a separate area.
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151
The IS-IS Hierarchy (continued)
Neighbors build their topology tables by sharing LSP’s (Link-State
Packets), which are roughly the equivalent of OSPF LSA’s. Depending on
the type of adjacency, a router will send out either a Level-1 or Level-2 LSP.
Level-1 routers share Level-1 LSP’s, and will build a Level-1 topology table
consisting of solely its own area (thus forming the equivalent of an OSPF
Totally Stubby area). If a Level-1 router has a packet destined for the local
area, it simply routes the packet to the System ID by using the local topology
table (Level-1 database).
If a Level-1 router has a packet destined for a remote area, it forwards it to
the nearest Level-1-2 router. Level-1-2 routers set an Attach (ATT) bit in
their Level-1 LSP’s, informing other Level-1 routers that they are attached
to another area.
Level-2 routers share Level-2 LSP’s, and will build a Level-2 topology
table, which contains a list of reachable areas across the IS-IS domain.
Level-1-2 routers will share both Level-1 and Level-2 LSP’s with its
appropriate adjacencies. Level-1-2 routers maintain separate Level-1 and
Level-2 topology tables.
Level-1 routes (locally originated) are always preferred over Level-2 routes
(originated from another area).
IS-IS routers will refresh the Link-State topology table every 15 minutes (as
opposed to every 30 minutes for OSPF).
(Reference: http://www.cisco.com/warp/public/cc/pd/iosw/prodlit/insys_wp.htm)
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152 Basic IS-IS Configuration
To configure IS-IS, the IS-IS process must first be established:
Router(config)# router isis
The router must then be configured with a CLNP address:
Router(config)# router isis Router(config-router)# net 49.0001.1921.6800.5005.00
To globally dictate the router-type of all interfaces (default is level-1-2):
Router(config)# router isis Router(config-router)# is-type level-1 Router(config-router)# is-type level-1-2 Router(config-router)# is-type level-2
Finally, IS-IS must be explicitly enabled on the interface:
Router(config)# interface fa0/0
Router(config-if)# ip router isis
This not only allows IS-IS to form neighbor relationships out of this
interface, it also adds the interface’s network to the routing table.
The globally configured router-type can be overridden on each individual
interface: Router(config)# interface fa0/0
Router(config-if)# isis circuit-type level-1
Router(config-if)# isis circuit-type level-1-2
Router(config-if)# isis circuit-type level-2
To adjust the priority (default is 64) of interface, increasing the likelihood
that the router will be elected the DIS:
Router(config)# interface e0/0
Router(config-if)# isis priority 100
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
153 IS-IS Passive-Interfaces
It is possible to control which router interfaces will participate in the IS-IS
process. Just as with EIGRP and OSPF, we can use the passive-interface
command.
However, please note that the passive-interface command works differently
with IS-IS than with RIP or IGRP. IS-IS will no longer form neighbor
relationships out of a “passive” interface, thus this command prevents
updates from being sent or received out of this interface:
RouterC(config)# router isis RouterC(config-router)# passive-interface s0
Router C will not form a neighbor adjacency with Router B.
We can configure all interfaces to be passive using the passive-interface
default command, and then individually use the no passive-interface
command on the interfaces we do want neighbors to be formed on:
RouterC(config)# router isis RouterC(config-router)# passive-interface default
RouterC(config-router)# no passive-interface e0
Always remember, that the passive-interface command will prevent IS-IS
(and OSPF) from forming neighbor relationships out of that interface. No
routing updates are passed in either direction.
However, unlike OSPF, using the passive-interface command will still
inject that interface’s network into the routing table. Thus, the passiveinterface
command can be useful when creating “stub” networks.
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consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
154 The IS-IS Metric
IS-IS utilizes an arbitrary cost for its metric (the optional metrics of delay,
expense, and error are not supported by Cisco). By default, interfaces of all
types (regardless of speed) are assigned a metric of 10.
To adjust the metric on an interface:
Router(config)# interface e0/0
Router(config-if)# isis metric 30
IS-IS Authentication
IS-IS authentication can be applied to a link, to an area, or to a domain.
Remember, Cisco supports only clear-text authentication for IS-IS.
To configuration authentication on an interface-by-interface basis:
Router(config)# interface fa0/0
Router(config-if)# isis password MYPASSWORD level-1
Router(config-if)# isis password MYPASSWORD2 level-2
Note that separate authentication passwords can be applied to Level-1 or
Level-2 Adjacencies. To configure authentication for an entire IS-IS area:
Router(config)# router isis Router(config-router)# area-password MYPASSWORD IS-IS Summarization
IS-IS supports both inter-area and external summarization, and uses the
same command to accomplish both. If we wished to summarize the
following networks into one summary route:
• 172.16.0.0/16 • 172.17.0.0/16 • 172.18.0.0/16 • 172.19.0.0/16 • 172.20.0.0/16 • 172.21.0.0/16 • 172.22.0.0/16 • 172.23.0.0/16
The following command would be required:
RouterC(config)# router isis
RouterC(config-router)# summary-address 172.16.0.0 255.248.0.0
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155
IS-IS and WAN Technologies
When configuring IS-IS over Frame-Relay, additional map statements are
required: Router(config)# interface s0/0
Router(config-if)# frame-relay map clns 105 broadcast
Router(config-if)# frame-relay map clns 106 broadcast
Additionally, we can map CLNP addresses in ISDN:
Router(config)# interface bri0
Router(config-if)# dialer map clns 49.0001.1921.6800.5005.00 name
MYNAME broadcast 3331111 IS-IS Troubleshooting
To view any CLNS neighbors, including the type of adjacency:
Router# show clns neighbors
To view only IS neighbors:
Router# show clns is-neighbors
To view specific IS-IS information about an interface:
Router# show clns interface e0/0
To view the IS-IS link-state topology table:
Router# show isis database
To view a list of all known IS-IS routers in all areas:
Router# show isis topology
(Reference: http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017d02e.html)
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156 IS-IS vs. OSPF
IS-IS is often compared and contrasted to OSPF. Both protocols share
several similarities, including:
• Both are Link-State routing protocols.
• Both use the Dijkstra algorithm to determine the shortest path.
• Both are classless and support VLSMs.
• Both use a cost metric.
• Both use areas to minimize the size of topology and routing tables.
• Both elect a designated router on broadcast links to contain link-state
update traffic.
Despite these similarities, there are a multitude of crucial differences
between IS-IS and OSPF, including:
• OSPF supports only IP, IS-IS supports both IP and CLNS.
• IS-IS does not require IP connectivity between routers to share routing
information. Updates are sent via CLNS instead of IP.
• In OSPF, interfaces belong to areas. In IS-IS, the entire router
belongs to an area.
• An IS-IS router belongs to only one Level-2 area, which results in less
LSP traffic. IS-IS is thus more efficient and scalable than OSPF, and
supports more routers per area.
• There is no Area 0 backbone area for IS-IS. The IS-IS backbone is a
contiguous group of Level 1-2 and Level 2 routers.
• IS-IS does not elect a backup DIS. Additionally, DIS election is
preemptive.
• On broadcast networks, even with an elected DIS, IS-IS routers still
form adjacencies with all other routers. In OSPF, routers will only
form adjacencies with the DR and BDR on broadcast links.
• IS-IS uses an arbitrary cost metric. OSPF’s cost metric is based on the
bandwidth of the link.
• IS-IS provides far more granular control of link-state and SPF timers
than OSPF. (Reference: http://geocities.com/mnvbhatia/draft-bhatia-manral-diff-isis-ospf-00.txt, http://www.ciscopress.com/articles/article.asp?p=31319&rl=1)
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
157 Section 14
- Border Gateway Protocol -
Border Gateway Protocol (BGP)
BGP is a standardized exterior gateway protocol (EGP), as opposed to RIP,
OSPF, and EIGRP which are interior gateway protocols (IGP’s). BGP
Version 4 (BGPv4) is the current standard deployment.
BGP is considered a “Path Vector” routing protocol. BGP was not built to
route within an Autonomous System (AS), but rather to route between AS’s.
BGP maintains a separate routing table based on shortest AS Path and
various other attributes, as opposed to IGP metrics like distance or cost.
BGP is the routing protocol of choice on the Internet. Essentially, the
Internet is a collection of interconnected Autonomous Systems.
BGP Autonomous Systems are assigned an Autonomous System Number
(ASN), which is a 16-bit number ranging from 1 – 65535. A specific subset
of this range, 64512 – 65535, has been reserved for private (or internal) use.
BGP utilizes TCP for reliable transfer of its packets, on port 179.
When to Use BGP
Contrary to popular opinion, BGP is not a necessity when multiple
connections to the Internet are required. Fault tolerance or redundancy of
outbound traffic can easily be handled by an IGP, such as OSPF or EIGRP.
BGP is also completely unnecessary if there is only one connection to an
external AS (such as the Internet). There are over 100,000 routes on the
Internet, and interior routers should not be needlessly burdened.
BGP should be used under the following circumstances:
• Multiple connections exist to external AS’s (such as the Internet) via
different providers.
• Multiple connections exist to external AS’s through the same
provider, but connect via a separate CO or routing policy.
• The existing routing equipment can handle the additional demands.
BGP’s true benefit is in controlling how traffic enters the local AS, rather
than how traffic exits it.
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158 BGP Peers (Neighbors)
For BGP to function, BGP routers (called speakers) must form neighbor
relationships (called peers).
There are two types of BGP neighbor relationships:
• iBGP Peers – BGP neighbors within the same autonomous system.
• eBGP Peers – BGP neighbors connecting separate autonomous
systems.
Note: Do not confuse an IGP, such as OSPF, with iBGP!
In the above figure, RouterB and RouterC in AS 200 would form an iBGP
peer relationship. RouterA in AS 100 and RouterB in AS 200 would form an
eBGP peering.
Once BGP peers form a neighbor relationship, they share their full routing
table. Afterwards, only changes to the routing table are forwarded to peers.
By default, BGP assumes that eBGP peers are a maximum of one hop away.
This restriction can be bypassed using the ebgp-multihop option with the
neighbor command (demonstrated later in this guide).
iBGP peers do not have a hop restriction, and are dependent on the
underlying IGP of the AS to connect peers together. By default, all iBGP
peers must be fully meshed within the Autonomous System.
A Cisco router running BGP can belong to only one AS. The IOS will only
allow one BGP process to run on a router.
The Administrative Distance for routes learned outside the Autonomous
System (eBGP routes) is 20, while the AD for iBGP and locally-originated
routes is 200.
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159 BGP Peers Messages
BGP forms its peer relationships through a series of messages. First, an
OPEN message is sent between peers to initiate the session. The OPEN
message contains several parameters:
• BGP Version – must be the same between BGP peers
• Local AS Number
• BGP Router ID
KEEPALIVE messages are sent periodically (every 60 seconds by default)
to ensure that the remote peer is still available. If a router does not receive a
KEEPALIVE from a peer for a Hold-time period (by default, 180 seconds),
the router declares that peer dead.
UPDATE messages are used to exchange routes between peers.
Finally, NOTIFICATION messages are sent when there is a fatal error
condition. If a NOTIFICATION message is sent, the BGP peer session is
torn down and reset.
As a BGP peer session is forming, it will pass through several states. This
process is known as the BGP Finite-State Machine (FSM):
• Idle – the initial BGP state
• Connect - BGP waits for a TCP connection with the remote peer. If
successful, an OPEN message is sent. If unsuccessful, the session is
placed in an Active state.
• Active – BGP attempts to initiate a TCP connection with the remote
peer. If successful, an OPEN message is sent. If unsuccessful, BGP
will wait for a ConnectRetry timer to expire, and place the session
back in a Connect State.
• OpenSent – BGP has both established the TCP connection and sent
an OPEN Message, and is awaiting a reply OPEN Message. Once it
receives a reply OPEN Message, the BGP peer will send a
KEEPALIVE message.
• OpenConfirm – BGP listens for a reply KEEPALIVE message.
• Established – the BGP peer session is fully established. UPDATE
messages containing routing information will now be sent.
If a peer session is stuck in an Active state, potential problems can include:
no IP connectivity (no route to host), an incorrect neighbor statement, or an
access-list filtering TCP port 179.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
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160 Configuring BGP Neighbors
The first step in configuring BGP is to enable the BGP process, and specify
the router’s Autonomous System (AS):
RouterB(config)# router bgp 100
RouterB is now a member of AS 100. Next, neighbor relationships must be
established. To configure a neighbor relationship with a router in the same
AS (iBGP Peer):
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 10.1.1.1 remote-as 100
To configure a neighbor relationship with a router in a separate AS (eBGP
Peer):
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 remote-as 900
Notice that the syntax is the same, and that the remote-as argument is always
used, regardless if the peering is iBGP or eBGP.
For stability purposes, the source interface used to generate updates to a
particular neighbor can be specified:
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 update-source lo0
RouterC must then point to RouterB’s loopback (assume the address is
1.1.1.1/24) in its neighbor statement:
RouterC(config)# router bgp 900
RouterC(config-router)# neighbor 1.1.1.1 remote-as 100
RouterC must have a route to RouterB’s loopback in its routing table.
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161
Configuring BGP Neighbors (continued)
Remember though: by default, BGP assumes that external peers are exactly
one hop away. Using the loopback as a source interface puts RouterB two
hops away from RouterC. Thus, the ebgp-multihop feature must be enabled:
RouterC(config)# router bgp 900
RouterC(config-router)# neighbor 1.1.1.1 ebgp-multihop 2
The 2 indicates the number of hops to the eBGP peer. If left blank, the
default is 255.
To authenticate updates between two BGP peers:
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 password CISCO
Configuring BGP Timers
To globally adjust the Keepalive and Hold-time timers for all neighbors:
RouterB(config)# router bgp 100
RouterB(config-router)# timers bgp 30 90
The above command sets the Keepalive timer to 30 seconds, and the Holdtime
timer to 90 seconds. If the configured Hold-time timers between two
peers are different, the peer session will still be established, and the smallest
timer value will be used.
To adjust the timers for a specific neighbor (which overrides the global timer
configuration):
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 timers 30 90
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162 Viewing BGP Neighbors
To view the status of all BGP neighbors:
RouterB# show ip bgp neighbors
BGP neighbor is 172.16.1.2, remote AS 900, external link
Index 1, Offset 0, Mask 0x2
Inbound soft reconfiguration allowed
BGP version 4, remote router ID 172.16.1.2
BGP state = Established, table version = 27, up for 00:03:45
Last read 00:00:19, hold time is 180, keepalive interval is 60
seconds
Minimum time between advertisement runs is 30 seconds
Received 25 messages, 0 notifications, 0 in queue
Sent 20 messages, 0 notifications, 0 in queue
Inbound path policy configured
Route map for incoming advertisements is testing
Connections established 2; dropped 1
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Local host: 172.16.1.1, Local port: 12342
Foreign host: 172.16.1.2, Foreign port: 179
Enqueued packets for retransmit: 0, input: 0, saved: 0
Event Timers (current time is 0x530C294):
Timer Starts Wakeups Next
Retrans 15 0 0x0
TimeWait 0 0 0x0
AckHold 15 13 0x0
SendWnd 0 0 0x0
KeepAlive 0 0 0x0
GiveUp 0 0 0x0
PmtuAger 0 0 0x0
<snip>
To view the status of a specific BGP neighbor:
RouterB# show ip bgp neighbors 172.16.1.2
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163 BGP Synchronization RouterA BGP AS 100 RouterD BGP RouterB BGP RouterC Non-BGP RouterE BGP 10.5.0.0/16
AS 200 AS 300
Consider the above example. AS 200 is serving as a transit between AS 100
and AS 300. BGP follows a synchronization rule that states that all routers
in a transit AS, including non-BGP routers, must learn of a route before BGP
can advertise it to an external peer.
Confused?
Consider the above example again. If RouterA advertises a BGP route to
RouterB (an eBGP peer) for the 10.5.0.0/16 network, that same BGP route
will eventually be forwarded to RouterD (an iBGP peer).
However, a blackhole would exist if RouterD then advertised that update to
RouterE, as RouterC would not have the 10.5.0.0/16 network in its routing
table. If RouterE attempts to reach the 10.5.0.0 network, RouterC will drop
the packet.
BGP’s synchronization rule will force RouterD to wait until RouterC learns
the 10.5.0.0/16 route, before forwarding that route to RouterE. How will
RouterD know when RouterC learns the route? Simple! When it receives an
update from RouterC via an IGP (such as OSPF), containing that route.
BGP synchronization can be disabled under two circumstances:
• The local AS is not a transit between two other AS’s
• All routers in the transit AS run iBGP, and are fully meshed.
To disable BGP synchronization:
RouterD(config)# router bgp 200
RouterD(config-router)# no synchronization
As of IOS 12.2(8)T, synchronization is disabled by default.
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164
Originating Prefixes in BGP
There are three ways to originate a prefix (in other words, advertise a
network) into BGP:
• By using network statements
• By using aggregate-address statements (explained later in this guide)
• By redistributing an IGP into BGP
Using the network statement informs BGP which networks to advertise to
eBGP peers, not which interfaces to run BGP on. The network command can
be used to inject any network from the local AS into BGP, include dynamic
routes learned from an IGP, and not just the routes directly connected to the
router.
However, the route must be in the routing table before BGP will advertise
the network to an eBGP peer. This is a fundamental BGP rule.
Consider the above example. RouterB may inject the 10.5.0.0/16 network
into BGP using the network command. However, unless that route is in the
local routing table (in this case, via an IGP), RouterB will not advertise the
route to RouterC.
Furthermore, the network statement must match the route exactly as it is
in the routing table:
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 remote-as 900
RouterB(config-router)# network 10.5.0.0 mask 255.255.0.0
The above configuration would match the route perfectly, while the
following configuration would not:
RouterB(config-router)# network 10.5.0.0 mask 255.255.255.0
If no mask is specified, a classful mask will be assumed.
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165
The BGP Routing Table
Recall that BGP maintains its own separate routing table. This table
contains a list of routes that can be advertised to BGP peers.
To view the BGP routing table on RouterB:
RouterB# show ip bgp
BGP table version is 426532, local router ID is 2.2.2.2
Status codes: s suppressed, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.5.0.0 0.0.0.0 0 0 32768 i
The route has been injected into BGP using the network command. The
Next Hop of 0.0.0.0 indicates that the route was locally originated into BGP.
The Path is empty, as the route originated in the Autonomous Systems.
Notice the Status Codes of “*>”. The * indicates that this route is valid (i.e.
in the routing table). The > indicates that this is the best route to the
destination.
BGP will never advertise a route to an eBGP peer unless it is both valid and
the best route to that destination. BGP routes that are both valid and best
will also added the IP routing table as well.
To view the BGP routing table on RouterC:
RouterC# show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 10.5.0.0 172.16.1.1 0 100 0 100 i
Notice that AS 100 has been added to the path, and that the Next Hop is
now RouterB.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
166 BGP Route-Reflectors
Recall that BGP requires all iBGP peers to be fully meshed. Route-
Reflectors allow us to bypass this restriction. Fewer neighbor connections
will result in less bandwidth and CPU usage.
Route-reflector clients form neighbor adjacencies with the route-reflector
server. BGP updates will flow from the server to the clients, without the
clients having to interact with each other.
Consider the above example. In AS 100, there are three BGP speakers.
Normally, these iBGP peers must be fully-meshed. For example, RouterB
would need a neighbor statement for both RouterA and RouterD.
As an alternative, RouterA can be configured as a route-reflector server.
Both RouterB and RouterD would only need to peer with RouterA.
All route-reflector specific configuration takes place on the route reflector
server:
RouterA(config)# router bgp 100
RouterA(config-router)# neighbor 10.2.1.2 remote-as 100
RouterA(config-router)# neighbor 10.2.1.2 route-reflector-client
RouterA(config-router)# neighbor 10.1.1.2 remote-as 100
RouterA(config-router)# neighbor 10.1.1.2 route-reflector-client
Route-reflectors are Cisco’s recommended method of alleviating the iBGP
full-mesh requirement.
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167 BGP Confederations RouterA RouterC AS 300 RouterB
AS 777 AS 888
AS 500
10.1.1.1 10.1.1.2 172.16.1.1 172.16.1.2
Confederations are an alternative method to alleviate the requirement that
all iBGP routers be fully meshed. Confederations are essentially AS’s
within an AS, and are sometimes referred to as sub-AS’s.
In the above example, RouterA belongs to AS 777 and RouterB belongs to
AS 888. Both of those AS’s belong to a parent AS of 300. RouterA and
RouterB will form an eBGP peer session.
Configuration is simple:
RouterB(config)# router bgp 888
RouterB(config-router)# bgp confederation identifier 300
RouterB(config-router)# bgp confederation peer 777
RouterB(config-router)# neighbor 10.1.1.1 remote-as 777
RouterB(config-router)# neighbor 172.16.1.2 remote-as 500
Notice that the sub-AS (777) is used in the router bgp statement.
Additionally, the parent AS must be specified using a bgp confederation
identifier statement. Finally, any confederation peers must be identified.
RouterC will be unaware of RouterB’s confederation status. Thus,
RouterC’s neighbor statement will point to AS 300, and not AS 888:
RouterC(config)# router bgp 500
RouterC(config-router)# neighbor 172.16.1.1 remote-as 300
(Reference: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#wp6834)
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168 BGP Peer-Groups
Peer-groups simplify configuration of groups of neighbors, assuming those
neighbors share identical settings. Additionally, peer-groups conserve
processor/memory resources by sending updates to all peer-group members
simultaneously, as opposed to sending individual updates to each neighbor.
All neighbor parameters are applied to the peer-group itself. Configuration is
simple:
Router(config)# router bgp 200
Router(config-router)# neighbor MYPEERGROUP peer-group
Router(config-router)# neighbor MYPEERGROUP remote-as 200
Router(config-router)# neighbor MYPEERGROUP update-source lo0
Router(config-router)# neighbor MYPEERGROUP route-reflector-client
The above configuration creates a peer-group named MYPEERGROUP, and
applies the desired settings. Next, we must “assign” the appropriate
neighbors to the peer-group:
Router(config-router)# neighbor 10.10.1.1 peer-group MYPEERGROUP
Router(config-router)# neighbor 10.10.2.2 peer-group MYPEERGROUP
Router(config-router)# neighbor 10.10.3.3 peer-group MYPEERGROUP
The above neighbors now inherit the settings of the peer-group named
MYPEERGROUP.
All “members” of a peer-group must exclusively be internal (iBGP) peers or
external (eBGP) peers. A mix of internal and external peers is not allowed in
a peer-group.
Outbound route filtering (via a distribution-list, route-map, etc.) must be
identical on all members of a peer-group. Inbound route filtering can still be
applied on a per-neighbor basis.
(Reference: http://www.cisco.com/warp/public/459/29.html)
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169 BGP Attributes
BGP utilizes several attributes to determine the best path to a destination.
Well-known attributes are supported by all implementations of BGP, while
optional attributes may not be supported by all BGP-speaking routers.
Several subcategories of attributes exist:
• Well-known Mandatory – Standard attributes supported by all BGP
implementations, and always included in every BGP update.
• Well-known Discretionary – Standard attributes supported by all
BGP implementations, and are optionally included BGP updates.
• Optional Transitive – Optional attribute that may not be supported
by all implementations of BGP. Transitive indicates that a noncompliant
BGP router will forward the unsupported attribute
unchanged, when sending updates to peers.
• Optional Non-Transitive - Optional attribute that may not be
supported by all implementations of BGP. Non-Transitive indicates
that a non-compliant BGP router will strip out the unsupported
attribute, when sending updates to peers.
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170 BGP Attributes (continued)
The following describes several specific BGP attributes:
• AS-Path (well-known mandatory) – Identifies the list (or path) of
traversed AS’s to reach a particular destination.
• Next-Hop (well-known mandatory) – Identifies the next hop IP
address to reach a particular destination.
• Origin (well-known mandatory) – Identifies the originator of the
route.
• Local Preference (well-known, discretionary) – Provides a
preference to determine the best path for outbound traffic.
• Atomic Aggregate (well-known discretionary) – Identifies routes
that have been summarized, or aggregated.
• Aggregator (optional transitive) – Identifies the BGP router that
performed an address aggregation.
• Community (optional transitive) – Tags routes that share common
characteristics into communities.
• Multi-Exit-Discriminator (MED) (optional non-transitive) –
Provides a preference to eBGP peers to a specific inbound router.
• Weight (Cisco Proprietary) – Similar to Local Preference, provides
a local weight to determine the best path for outbound traffic.
Each attribute is identified by a code:
Origin AS-Path Next Hop MED Local Preference Automatic Aggregate Aggregator Community Code 1 Code 2 Code 3 Code 4 Code 5 Code 6 Code 7 Code 8
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171
BGP “Best Path” Determination
If BGP contains multiple routes to the same destination, it compares the
routes in pairs, starting with the newest entries (listed higher in the routing
table), and working towards the oldest entries (listed lower in the table).
BGP determines the best path by successively comparing the attributes of
each “route pair.” The attributes are compared in a specific order:
• Weight – Which route has the highest weight?
• Local Preference – Which route has the highest local preference?
• Locally Originated – Did the local router originate this route? In
other words, is the next hop to the destination 0.0.0.0?
• AS-Path – Which route has the shortest AS-Path?
• Origin Code – Where did the route originate? The following origin
codes are listed in order of preference:
o IGP (originated from an interior gateway protocol)
o EGP (originated from an exterior gateway protocol)
o ? (Unknown origin)
• MED – Which path has the lowest MED?
• BGP Route Type – Is this an eBGP or iBGP route? (eBGP routes are
preferred)
• Age – Which route is the oldest? (oldest is preferred)
• Router ID – Which route originated from the router with the lowest
BGP router ID?
• Peer IP Address – Which route originated from the router with the
lowest IP?
When applying attributes, Weight and Local Preference are applied to
inbound routes, dictating the best outbound path.
AS-Path and MED are applied to outbound routes, dictating the best inbound
path. (Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml)
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172 Weight
The Weight attribute is applied to inbound routes, dictating the best
outbound path. It is a Cisco-proprietary attribute, and is only locally
significant (and thus, is never passed on to BGP neighbors).
The weight value can range from 0 – 65535, and the highest weight is
preferred. By default, a route originated on the local router will be assigned a
weight of 32768. All other routes will be assigned a weight of 0, by default.
A weight value can be specified for all routes advertised from a specific
neighbor:
RouterA(config)# router bgp 100
RouterA(config)# neighbor 10.1.1.2 weight 200
Otherwise, a weight value can be specified for specific routes from a
particular neighbor. First, the prefixes in question must be identified:
RouterA(config)# ip prefix-list MYLIST 192.168.1.0/24
Then, a route-map is used to apply the appropriate weight:
RouterA(config)# route-map WEIGHT permit 10
RouterA(config-route-map)# match ip address prefix-list MYLIST
RouterA(config-route-map)# set weight 200
RouterA(config-route-map)# route-map WEIGHT permit 20
Finally, the route-map is applied to the preferred neighbor:
RouterA(config)# router bgp 100
RouterA(config)# neighbor 10.1.1.2 route-map WEIGHT in
(Reference: http://www.cisco.com/warp/public/459/bgp-toc.html#weight)
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
173 Local Preference
The Local Preference attribute is applied to inbound external routes,
dictating the best outbound path. Unlike the Weight attribute, Local
Preference is passed on to iBGP peers when sending updates. Local
Preference informs iBGP routers how to exit the AS, if multiple paths exist.
Local Preference is a 32-bit number, and can range from 0 to 4294967295.
The highest Local Preference is preferred, and the default preference is 100.
The Local Preference value can be specified for all inbound external routes,
on a global basis for BGP:
RouterB(config)# router bgp 100
RouterB(config-router)# bgp default local-preference 200
RouterD(config)# router bgp 100
RouterD(config-router)# bgp default local-preference 300
Both RouterB and RouterD will include the Local Preference attribute in
updates to iBGP neighbors. Thus, RouterA (and RouterB) will now prefer
the route through RouterD to reach any destination outside the local AS.
Local Preference can be applied on a per-route basis:
RouterD(config)# ip prefix-list MYLIST 192.168.1.0/24
RouterD(config)# route-map PREFERENCE permit 10
RouterD(config-route-map)# match ip address prefix-list MYLIST
RouterD(config-route-map)# set local-preference 300
RouterD(config)# router bgp 10
RouterD(config)# neighbor 172.17.1.2 route-map PREFERENCE in
(Reference: http://www.cisco.com/warp/public/459/bgp-toc.html#localpref)
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174 AS-Path Prepend
The AS-Path attribute is applied to outbound routes, dictating the best
inbound path. Two things can be accomplished with the AS-Path attribute,
prepend or filter.
To prepend to (or add to) the existing AS-Path results in a longer AS-Path,
which makes the route less desirable for inbound traffic:
RouterB(config)# access-list 5 permit 10.5.0.0 0.0.255.255
RouterB(config)# route-map ASPREPEND permit 10
RouterB(config-route-map)# match ip address 5
RouterB(config-route-map)# set as-path prepend 200 200
RouterB(config-route-map)# route-map ASPREPEND permit 20
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 route-map ASPREPEND out
The artificial AS-Path information is not added to a route until it is
advertised to an eBGP peer. RouterC’s BGP routing table will now look as
follows:
RouterC# show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 10.5.0.0 172.16.1.1 0 100 0 100 200 200 i
*> 10.5.0.0 172.17.1.1 0 100 0 100 i
Notice the inflated AS-Path through RouterB. RouterC will prefer the path
through RouterD to reach the 10.5.0.0/16 network.
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175 AS-Path Filtering
Additionally, routes can be filtered based on AS-Path values, using an aspath
access-list. This requires the use of regular expressions:
• ^ = Start of a string
• $ = End of a string
• . = Any one character
• * = Any one or more characters, including none
• + = Any one or more characters
• ? = Any one character, including none
• _ = Serves the function of virtually all of the above
The following examples illustrate the use of regular expressions:
• ^100_ = learned from AS 100
• _100$ = originated from AS 100
• ^$ = originated locally
• .* = matches everything
• _100_ = any instance of AS 100
To configure RouterF to only accept routes that originated from AS100:
RouterF(config)# ip as-path access-list 15 permit _100$
RouterF(config)# route-map ASFILTER permit 10
RouterF(config-route-map)# match as-path 15
RouterF(config)# router bgp 50
RouterF(config-router)# neighbor 10.5.1.1 route-map ASFILTER in
To view what BGP routing entries the AS-Path access-list will match:
RouterF# show ip bgp regexp _100$
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a92.shtml)
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176 Origin
The Origin attribute identifies the originating source of the route. The
origin codes are as follows (listed in order of preference for route selection):
• i (IGP) – Originated from an interior gateway protocol, such as
OSPF. This usually indicates the route was injected into BGP via the
network command under the BGP process. An origin code of “i” is
most preferred.
• e (EGP) – Originated from an external gateway protocol.
• ? (incomplete) - Unknown origin. This usually indicates the route
was redistributed into BGP (from either connected, static, or IGP
routes). An origin code of “?” is the least preferred.
When viewing the BGP routing table, the origin code is listed at the end of
each line in the table:
RouterB# show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 10.5.0.0 10.1.1.1 0 0 0 i
*> 192.168.1.0 172.16.1.2 0 100 0 900 ?
The i at the end of the first routing entry indicates the 10.5.0.0 network was
originated via an IGP, probably with the BGP network command. The
192.168.1.0 network was most likely redistributed into BGP in AS 900, as
evidenced by the ? at the end of that routing entry.
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177 MED
The MED (MultiExit Discriminator) attribute is applied to outbound
routes, dictating the best inbound path into the AS (assuming multiple paths
exist). The MED is identified as the BGP metric when viewing the BGP
routing table. A lower metric is preferred, and the default MED value is 0.
In the above example, there are two entry points into AS 100. To force AS
900 to prefer that path through RouterD to reach the 10.5.0.0/16 network,
the set metric command can be used with a route-map:
RouterB(config)# access-list 5 permit 10.5.0.0 0.0.255.255
RouterB(config)# route-map SETMED permit 10
RouterB(config-route-map)# match ip address 5
RouterB(config-route-map)# set metric 200
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 route-map SETMED out
RouterC will now have two entries for the 10.5.0.0/16 route:
RouterC# show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 10.5.0.0 172.16.1.1 200 100 0 100 i
*> 10.5.0.0 172.17.1.1 0 100 0 100 i
Notice that the route from RouterB has a higher metric, and thus is less
preferred. Note specifically the lack of a > on the route with a higher metric.
The MED value is exchanged from one AS to another, but will never be
advertised further than that. Thus, the MED value is passed from AS 100 to
all BGP routers in AS 900, but the metric will be reset to 0 if the route is
advertised beyond AS 900.
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178 MED (continued)
A key aspect to consider when using the MED attribute is BGP’s method of
route selection. Recall that if BGP contains multiple routes to the same
destination, it compares the routes in pairs, starting with the newest entries
and working towards the oldest entries.
This can lead to sub-optimal routing, depending on the order of routes in the
BGP routing table. BGP employs two MED-related commands to alleviate
potential sub-optimal routing selections.
The bgp deterministic-med command forces the MED value to be compared,
when multiple routes to the same network are received via multiple routers
from the same AS, regardless of the order of routes in the BGP routing table.
RouterE(config)# router bgp 100
RouterE(config-router)# bgp deterministic-med
The bgp deterministic-med command is disabled by default. If used, the
command should be enabled on all routers within the AS.
The bgp always-compare-med command forces the MED value to be
compared, when multiple routes to the same network are received via
multiple routers from different AS’s, regardless of the order of routes in the
BGP routing table.
RouterE(config)# router bgp 100
RouterE(config-router)# bgp always-compare-med
The bgp always-compare-med command is disabled by default. Thus, by
default, the MED value is not compared between paths from different AS’s.
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml; http://www.cisco.com/warp/public/459/bgp-toc.html#metricattribute)
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179 MED (continued)
The MED metric on routes sent to eBGP neighbors can be dynamically set
to the actual metric of an IGP (such as OSPF). This is accomplished using
the set metric-type internal command with a route-map:
RouterB(config)# access-list 5 permit 10.5.0.0 0.0.255.255
RouterB(config)# route-map MED_INTERNAL permit 10
RouterB(config-route-map)# match ip address 5
RouterB(config-route-map)# set metric-type internal
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.17.1.2 route-map MED_INTERNAL out
If the 10.5.0.0/16 network originated in OSPF, the link-state cost metric for
that route will be applied as the MED metric.
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180 Communities
BGP allows routes to be placed (or tagged) into certain Communities. BGP
routers can make route policy decisions based on a route’s community
membership.
BGP communities can be assigned using one of three 32-bit formats:
• Decimal (1000000) • Hexadecimal (0x1A2B3C) • AA:NN (100:20)
The AA:NN format specifies a 16-bit AS number (the AA), and a 16-bit
generic community identifier (NN).
By default, the decimal format for communities will be displayed when
viewing a route. To force the router to display the AA:NN format:
RouterA(config)# ip bgp-community new-format
Additionally, there are four well-known communities that can be referenced
by name:
• No-export – prevents the route from being advertised outside the
local AS to eBGP peers.
• No-advertise – prevents the route from being advertised to either
internal or external peers.
• Internet – allows the route to be advertised outside the local AS.
• Local-AS – prevents the route from being advertised outside the
local AS to either eBGP or confederate peers.
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a00800949e8.shtml#four; http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#communityattribute)
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181 Communities (continued)
To set the community for a specific route, using a route-map:
RouterB(config)# access-list 5 permit 10.5.0.0 0.0.255.255
RouterB(config)# route-map COMMUNITY permit 10
RouterB(config-route-map)# match ip address 5
RouterB(config-route-map)# set community no-export
RouterB(config)# route-map COMMUNITY permit 20
RouterB(config)# router bgp 100
RouterB(config-router)# neighbor 172.16.1.2 send-community
RouterB(config-router)# neighbor 172.16.1.2 route-map COMMUNITY out
The community attribute will not be advertised to a neighbor unless the
send-community parameter is applied to the neighbor command, regardless
if a community value is applied using a route-map.
The above configuration will place the 10.5.0.0/16 route into the no-export
community once it is advertised into AS 900. RouterC will advertise this
network to all iBGP peers, but the community attribute will prevent
RouterC (and all iBGP peers) from advertising the route outside of AS 900.
By default, the set community route-map command will overwrite any
existing community parameters for a route. To instead append additional
community values, the additive parameter must be specified:
RouterB(config)# route-map COMMUNITY permit 10
RouterB(config-route-map)# match ip address 5
RouterB(config-route-map)# set community no-export additive
RouterB(config)# route-map COMMUNITY permit 20
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182 BGP Summarization
Routes that are redistributed into BGP are automatically summarized. To
disable auto-summary:
Router(config)# router bgp 100
Router(config-router)# no auto-summary
To manually create a summary address for the following group of networks:
• 172.16.0.0/24 • 172.16.1.0/24 • 172.16.2.0/24 • 172.16.3.0/24
The aggregate-address command must be used:
Router(config)# router bgp 100
Router(config-router)# aggregate-address 172.16.0.0 255.255.252.0
BGP’s default configuration is to send both the summary (or aggregated)
address and the more specific individual routes. To only send the summary
route:
Router(config)# router bgp 100
Router(config-router)# aggregate-address 172.16.0.0 255.255.252.0 summaryonly
To suppress (or summarize) only specific routes, instead of all routes, a
route-map must be used:
Router(config)# access-list 5 permit 172.16.0.0 0.0.0.255
Router(config)# access-list 5 permit 172.16.1.0 0.0.0.255
Router(config)# route-map SUPPRESS permit 10
Router(config-route-map)# match ip address 5
Router(config)# router bgp 100
Router(config-router)# aggregate-address 172.16.0.0 255.255.252.0 summaryonly
suppress-map SUPPRESS
The access-list details the routes that should be suppressed. To allow the
summarized routes to retain their AS-Path information:
Router(config)# router bgp 100
Router(config-router)# aggregate-address 172.16.0.0 255.255.252.0 summaryonly
suppress-map SUPPRESS as-set
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183 BGP Route Dampening
Route dampening “suppresses” routes that are flapping, minimizing
unnecessary convergence and updates. If a route flaps (goes up and down),
it is assigned a penalty (default is 1000). All routes start with a penalty of 0,
and the local router maintains a history of routes that have flapped.
Once the penalty reaches a specific threshold, the route is suppressed. When
a route is suppressed, it is neither advertised nor used locally on the router.
First, the routes to be “observed” must be identified using an access-list or
prefix-list:
Router(config)# ip prefix-list MYLIST seq 10 permit 10.1.0.0/16
Router(config)# ip prefix-list MYLIST seq 20 permit 10.2.0.0/16
Next, dampening values must be configured using a route-map:
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address prefix-list MYLIST
Router(config-route-map)# set dampening 15 750 2000 60
The above values for the set dampening command represent the defaults.
The 15 (measured in minutes) indicates the half-life timer. If a route is
assigned a penalty, half of the penalty will decay after this timer expires.
The 750 (arbitrary penalty measurement) indicates the bottom threshold.
Once a penalized route falls below this threshold, it will no longer be
suppressed.
The 2000 (arbitrary penalty measurement) indicates the top threshold. If a
route flaps to the point that its penalty exceeds this threshold, it is
suppressed.
The 60 (measured in minutes) indicates the maximum amount of time a
route can be suppressed.
Finally, route-dampening must be enabled under the BGP process:
Router(config)# router bgp 100
Router(config-router)# bgp dampening route-map MYMAP
(Reference: http://www.cisco.com/warp/public/459/bgp-rec-routing.html)
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184 BGP Next-Hop-Self
Consider the above diagram. If RouterC sends the 192.168.1.0/24 route to its
eBGP peer RouterB, the Next Hop for that route will be through RouterC:
RouterB# show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.168.1.0 172.16.1.2 0 100 0 900 i
A serious problem arises when RouterB sends this route to its iBGP peers
(RouterA and RouterD). The Next Hop value is not changed:
RouterA# show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 192.168.1.0 172.16.1.2 0 100 0 900 i
Notice the lack of >, indicating this is no longer the best route to the
destination. This is because RouterA has no route to the next hop address.
There are two workarounds. Either the 172.16.0.0/16 network must be added
to RouterA’s and RouterD’s routing tables, or the Next-Hop field must be
adjusted to identify RouterB as the next hop.
The configuration is simple, and is completed on RouterB:
RouterB(config)# router bgp 200
RouterB(config-router)# neighbor 10.1.1.1 next-hop-self
RouterB(config-router)# neighbor 10.2.1.2 next-hop-self
RouterB now advertises itself as the next hop for all eBGP routes it learns:
RouterA# show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.168.1.0 10.1.1.2 0 100 0 900 i
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185 BGP Backdoor
Recall that an external BGP route has an Administrative Distance (AD) of
20, which is less than the default AD of IGP’s, such as OSPF or EIGRP.
Under certain circumstances, this may result in sub-optimal routing. If both
an IGP route and eBGP route exist to the same network, and the IGP route
should be preferred, there are two workarounds:
• Globally change BGP’s default Administrative Distance values.
• Use the BGP network backdoor command.
Cisco does not recommend changing BGP’s default AD values. If necessary,
however, the distance bgp will adjust the AD for external, internal, and
locally-originated BGP routes, respectively:
Router(config)# router bgp 100
Router(config-router)# distance bgp 150 210 210
The preferred workaround is to use the BGP network backdoor command,
which adjusts the AD for a specific eBGP route (by default, from 20 to 200),
resulting in the IGP route being preferred:
Router(config)# router bgp 100
Router(config-router)# network 10.5.0.0 mask 255.255.0.0 backdoor
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#bgpbackdoor)
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186 Misc. BGP Commands
To restrict the number of routes a BGP router can receive from its neighbor:
Router(config)# router bgp 200
Router(config-router)# neighbor 10.1.1.1 maximum-prefix 10000
To immediately reset an eBGP session if a link connecting two peers goes
down, the bgp fast-external-fallover feature must be enabled. To enable this
feature globally:
Router(config)# router bgp 200
Router(config-router)# bgp fast-external-fallover
To enable this feature on a per-interface basis:
Router(config)# int serial0/0
Router(config-if)# ip bgp fast-external-fallover permit
To reset the BGP session between all neighbors:
Router# clear ip bgp *
To force a resend of routing updates, without resetting any BGP sessions
between neighbors:
Router# clear ip bgp * soft
To view a summary of all BGP connections, including the total number of
BGP routes and a concise list of neighbors:
Router# show ip bgp summary
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187 ________________________________________________ Part IV Advanced Routing Functions ________________________________________________
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188 Section 15
- Route Redistribution -
Route Redistribution Basics
It is preferable to employ a single routing protocol in an internetwork
environment, for simplicity and ease of management. Unfortunately, this is
not always possible, making multi-protocol environments common.
Route Redistribution allows routes from one routing protocol to be
advertised into another routing protocol. The routing protocol receiving
these redistributed routes usually marks the routes as external. External
routes are usually less preferred than locally-originated routes.
At least one redistribution point needs to exist between the two routing
domains. This device will actually run both routing protocols. Thus, to
perform redistribution in the following example, RouterB would require at
least one interface in both the EIGRP and the OSPF routing domains:
It is possible to redistribute from one routing protocol to the same routing
protocol, such as between two separate OSPF domains (distinguished by
unique process ID’s). Static routes and connected interfaces can be
redistributed into a routing protocol as well.
Routes will only be redistributed if they exist in the routing table. Routes
that are simply in a topology database (for example, an EIGRP Feasible
Successor), will never be redistributed.
Routing metrics are a key consideration when performing route
redistribution. With the exception of IGRP and EIGRP, each routing
protocol utilizes a unique (and thus incompatible) metric. Routes
redistributed from the injecting protocol must be manually (or globally)
stamped with a metric that is understood by the receiving protocol.
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189 Redistributing into RIP
RIP is a standardized Distance-Vector routing protocol that uses hop-count
as its distance metric. Consider the following example:
RouterB is our redistribution point between IGRP and RIP. To redistribute
all IGRP routes into RIP:
RouterB(config)# router rip RouterB(config-router)# network 172.16.0.0
RouterB(config-router)# redistribute igrp 10 metric 2
First, the router rip process was enabled. Next, RIP was configured to
advertise the network of 172.16.0.0/16. Finally, RIP was configured to
redistribute all igrp routes from Autonomous System 10, and apply a hopcount
metric of 2 to the redistributed routes. If a metric is not specified, RIP
will assume a metric of 0, and will not advertise the redistributed routes.
Redistributing into IGRP
IGRP is a Cisco-proprietary Distance-Vector routing protocol that, by
default, uses a composite of bandwidth and delay as its distance metric.
IGRP can additionally consider Reliability, Load, and MTU for its metric.
Still using the above example, to redistribute all RIP routes into IGRP:
RouterB(config)# router igrp 10
RouterB(config-router)# network 10.0.0.0
RouterB(config-router)# redistribute rip metric 10000 1000 255 1 1500
First, the router igrp process was enabled for Autonomous System 10. Next,
IGRP was configured to advertise the network of 10.0.0.0/8. Finally, IGRP
was configured to redistribute all rip routes, and apply a metric of 10000
(bandwidth), 1000 (delay), 255 (reliability), 1 (load), and 1500 (MTU) to the
redistributed routes.
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190 Redistributing into EIGRP
EIGRP is a Cisco-proprietary hybrid routing protocol that, by default, uses a
composite of bandwidth and delay as its distance metric. EIGRP can
additionally consider Reliability, Load, and MTU for its metric.
To redistribute all OSPF routes into EIGRP:
RouterB(config)# router eigrp 15
RouterB(config-router)# network 10.1.2.0 0.0.0.255
RouterB(config-router)# redistribute ospf 20 metric 10000 1000 255 1 1500
First, the router eigrp process was enabled for Autonomous System 15.
Next, EIGRP was configured to advertise the network of 10.1.2.0/24.
Finally, EIGRP was configured to redistribute all ospf routes from process-
ID 20, and apply a metric of 10000 (bandwidth), 1000 (delay), 255
(reliability), 1 (load), and 1500 (MTU) to the redistributed routes.
It is possible to specify a default-metric for all redistributed routes:
RouterB(config)# router eigrp 15
RouterB(config-router)# redistribute ospf 20
RouterB(config-router)# default-metric 10000 1000 255 1 1500
RIP and IGRP also support the default-metric command. Though
IGRP/EIGRP use only bandwidth and delay by default to compute the
metric, it is still necessary to specify all five metrics when redistributing. If
the default-metric or a manual metric is not specified, IGRP/EIGRP will
assume a metric of 0, and will not advertise the redistributed routes.
Redistribution will occur automatically between IGRP and EIGRP on a
router, if both processes are using the same Autonomous System number.
EIGRP, by default, will auto-summarize internal routes unless the no autosummary
command is used. However, EIGRP will not auto-summarize
external routes unless a connected or internal EIGRP route exists in the
routing table from the same major network of the external routes.
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191 Redistributing into OSPF
OSPF is a standardized Link-State routing protocol that uses cost (based on
bandwidth) as its link-state metric. An OSPF router performing
redistribution automatically becomes an ASBR.
To redistribute all EIGRP routes into OSPF:
RouterB(config)# router ospf 20
RouterB(config-router)# network 172.16.0.0 0.0.255.255 area 0
RouterB(config-router)# redistribute eigrp 15
RouterB(config-router)# default-metric 30
First, the router ospf process was enabled with a process-ID of 20. Next,
OSPF was configured to place any interfaces in the network of 172.16.0.0/16
into area 0. Then, OSPF will redistribute all eigrp routes from AS 15.
Finally, a default-metric of 30 was applied to all redistributed routes.
If the default-metric or a manual metric is not specified for the redistributed
routes, a default metric of 20 will be applied to routes of all routing
protocols except for BGP. Redistributed BGP routes will have a default
metric of 1 applied by OSPF.
By default, OSPF will only redistribute classful routes into the OSPF
domain. To configure OSPF to accept subnetted networks during
redistribution, the subnets parameter must be used:
RouterB(config)# router ospf 20
RouterB(config-router)# redistribute eigrp 15 subnets
Routes redistributed into OSPF are marked external. OSPF identifies two
types of external routes, Type-1 (which is preferred) and Type-2 (which is
default). To change the type of redistributed routes:
RouterB(config)# router ospf 20
RouterB(config-router)# redistribute eigrp 15 subnets metric-type 1
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192
Redistributing Static and Connected Routes
Redistributing static routes into a routing protocol is straightforward:
RouterB(config)# router eigrp 15
RouterB(config-router)# redistribute static
Redistributing networks on connected interfaces into a routing protocol is
equally straightforward:
RouterB(config)# router eigrp 15
RouterB(config-router)# redistribute connected
The above commands redistribute all connected networks into EIGRP.
Route-maps can be used to provide more granular control:
RouterB(config)# route-map CONNECTED permit 10
RouterB(config-route-map)# match interface fa0/0, fa0/1, s0/0, s0/1
RouterB(config)# router eigrp 15
RouterB(config-router)# redistribute connected route-map CONNECTED
Connected networks can be indirectly redistributed into a routing protocol.
Recall that routes will only be redistributed if they exist in the routing table,
and consider again the following example:
If RouterB is configured as follows:
RouterB(config)# router eigrp 15
RouterB(config-router)# network 10.1.2.0 0.0.0.255
RouterB will advertise the 10.1.2.0/24 network to RouterA, but it will not
have an EIGRP route in its routing table for that network, as the network is
directly connected.
Despite this, when redistributing EIGRP into OSPF, the 10.1.2.0/24 is still
injected into OSPF. The network 10.1.2.0 0.0.0.255 command under the
EIGRP process will indirectly redistribute this network into OSPF.
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193
Pitfalls of Route Redistribution – Administrative Distance
Route redistribution introduces unique problems when there are multiple
points of redistribution. Consider the following diagram:
The first issue is caused by Administrative Distance (AD), which
determines which routing protocol is “trusted” the most. By default, OSPF
routes have an AD of 110, whereas RIP routes have an AD of 120. Lowest
AD is preferred, thus making the OSPF routes the most trusted.
Assume mutual redistribution has been performed on RouterC and RouterD.
The following networks will be injected from RIP into OSPF: 10.1.1.0/24,
10.1.2.0/24, 10.1.3.0/24, 10.1.4.0/24, and 10.1.5.0/24.
RouterC will eventually receive OSPF routes to the above networks from
RouterD, in addition to the RIP routes already in its table. Likewise,
RouterD will receive OSPF routes to these networks from RouterC.
Because OSPF’s AD is lower than RIP’s, both RouterC and RouterD will
prefer the sub-optimal path through OSPF to reach the non-connected
networks. Thus, RouterC will choose the OSPF route for all the 10.x.x.x/24
networks except for 10.1.1.0/24, as it is already directly connected.
This actually creates a routing loop. RouterC will prefer the OSPF path
through RouterA to reach the 10.x.x.x networks (except for 10.1.1.0/24), and
RouterA will likely consider RouterC its shortest path to reach those same
networks. Traffic will be continuously looped between these two routers.
Even if RouterC managed to send the traffic through RouterA and RouterB
to RouterD, the preferred path to the 10.x.x.x networks for RouterD is still
through OSPF. Thus, the routing loop is inevitable.
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194
Pitfalls of Route Redistribution – Administrative Distance (continued)
There are two methods to correct this particular routing loop. The first
method involves filtering incoming routes using a distribution-list,
preventing RouterC and RouterD from accepting any routes that originated
in RIP from their OSPF neighbors.
RouterC’s configuration would be as follows:
RouterC(config)# access-list 10 deny 10.1.2.0 0.0.0.255
RouterC(config)# access-list 10 deny 10.1.3.0 0.0.0.255
RouterC(config)# access-list 10 deny 10.1.4.0 0.0.0.255
RouterC(config)# access-list 10 deny 10.1.5.0 0.0.0.255
RouterC(config)# access-list 10 permit any
RouterC(config)# router ospf 20
RouterC(config-router)# distribute-list 10 in fastethernet0/0
An access-list was created that is denying the RIP networks in question, and
permitting all other networks. Under the OSPF process, a distribute-list is
created for routes coming inbound off of the fastethernet0/0 interface. The
access-list and distribute-list numbers must match. RouterD’s configuration
would be similar.
This prevents each router from building OSPF routes for the networks that
originated in RIP, and thus eliminates the possibility of a loop. However,
redundancy is also destroyed – if RouterC’s fa0/1 interface were to fail, it
could not choose the alternate path through OSPF.
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195
Pitfalls of Route Redistribution – Administrative Distance (continued)
The second method involves using the distance command to adjust the AD
of specific routes. This can accomplished two ways:
• Lowering the AD of the local RIP-learned routes
• Raising the AD of the external OSPF-learned routes
To force the RIP routes to be preferred, RouterC’s configuration would be as
follows:
RouterC(config)# access-list 10 permit 10.1.2.0 0.0.0.255
RouterC(config)# access-list 10 permit 10.1.3.0 0.0.0.255
RouterC(config)# access-list 10 permit 10.1.4.0 0.0.0.255
RouterC(config)# access-list 10 permit 10.1.5.0 0.0.0.255
RouterC(config)# access-list 10 deny any
RouterC(config)# router rip
RouterC(config-router)# distance 70 10.1.1.0 0.0.0.255 10
An access-list was created that is permitting the RIP networks in question,
and denying all other networks. Under the RIP process, an administrative
distance of 70 is applied to updates from routers on the 10.1.1.0 network, for
the specific networks matching access-list 10. RouterD’s configuration
would be similar.
Thus, the RIP-originated networks will now have a lower AD than the
redistributed routes from OSPF. The loop has again been eliminated.
Another solution would be to raise the AD of the external OSPF routes.
OSPF provides a simple mechanism to accomplish this:
RouterC(config)# router ospf 20
RouterC(config-router)# distance ospf external 240
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196
Pitfalls of Route Redistribution – Route Feedback
A routing loop is only one annoying issue resulting from the above design.
Route feedback is another problem that must be addressed.
OSPF routes redistributed into RIP on RouterC will eventually reach
RouterD, and then be redistributed again back into OSPF. This is a basic
example of route feedback.
Depending on the metrics used, this could potentially cause RouterB to
prefer the route through RouterD (and through the RIP domain), to reach the
192.168.2.0/24 network. This is an obvious example of suboptimal routing.
Thus, routes that originated in a routing domain should not to be re-injected
into that domain. Distribution-lists and the distance command can be utilized
to accomplish this, but route tags may provide a more robust solution.
Tagging routes provides a mechanism to both identify and filter those routes
further along in the routing domain. A route retains its tag as it passes from
router to router. Thus, if a route is tagged when redistributed into RIP on
RouterC, that same route can be selectively filtered once it is advertised to
RouterD.
Route tags are applied using route-maps. Route-maps provide a sequential
list of commands, each having a permit or deny result:
RouterC(config)# route-map OSPF2RIP deny 5
RouterC(config-route-map)# match tag 33
RouterC(config-route-map)# route-map OSPF2RIP permit 15
RouterC(config-route-map)# set tag 44
Route-maps are covered in great detail in a separate guide.
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197
Pitfalls of Route Redistribution – Route Feedback (continued)
The full configuration necessary on RouterC would be as follows:
RouterC(config)# route-map OSPF2RIP deny 5
RouterC(config-route-map)# match tag 33
RouterC(config-route-map)# route-map OSPF2RIP permit 15
RouterC(config-route-map)# set tag 44
RouterC(config)# router rip
RouterC(config)# redistribute ospf 20 route-map OSPF2RIP
RouterC(config)# route-map RIP2OSPF deny 5
RouterC(config-route-map)# match tag 44
RouterC(config-route-map)# route-map OSPF2RIP permit 15
RouterC(config-route-map)# set tag 33
RouterC(config)# router ospf 20
RouterC(config)# redistribute rip route-map RIP2OSPF
Thus, OSPF routes being redistributed into RIP are set with a tag of 44.
When RIP is redistributed back into OSPF, any route with a tag that matches
44 is denied.
Similarly, RIP routes being redistributed into OSPF are set with a tag of 33.
When OSPF is redistributed back into RIP, any route with a tag that matches
33 is denied.
The net result: routes originating from a routing domain will not
redistributed back into that domain.
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198 Section 16
- Access Control Lists -
Access Control Lists (ACLs)
Access control lists (ACLs) can be used for two purposes on Cisco devices:
• To filter traffic
• To identify traffic
Access lists are a set of rules, organized in a rule table. Each rule or line in
an access-list provides a condition, either permit or deny:
• When using an access-list to filter traffic, a permit statement is used to
“allow” traffic, while a deny statement is used to “block” traffic.
• Similarly, when using an access list to identify traffic, a permit
statement is used to “include” traffic, while a deny statement states
that the traffic should “not” be included. It is thus interpreted as a
true/false statement.
Filtering traffic is the primary use of access lists. However, there are several
instances when it is necessary to identify traffic using ACLs, including:
• Identifying interesting traffic to bring up an ISDN link or VPN tunnel
• Identifying routes to filter or allow in routing updates
• Identifying traffic for QoS purposes
When filtering traffic, access lists are applied on interfaces. As a packet
passes through a router, the top line of the rule list is checked first, and the
router continues to go down the list until a match is made. Once a match is
made, the packet is either permitted or denied.
There is an implicit ‘deny all’ at the end of all access lists. You don’t create
it, and you can’t delete it. Thus, access lists that contain only deny
statements will prevent all traffic.
Access lists are applied either inbound (packets received on an interface,
before routing), or outbound (packets leaving an interface, after routing).
Only one access list per interface, per protocol, per direction is allowed.
More specific and frequently used rules should be at the top of your access
list, to optimize CPU usage. New entries to an access list are added to the
bottom. You cannot remove individual lines from a numbered access list.
You must delete and recreate the access to truly make changes. Best practice
is to use a text editor to manage your access-lists.
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199
Types of Access Lists
There are two categories of access lists: numbered and named.
Numbered access lists are broken down into several ranges, each dedicated
to a specific protocol:
1–99 IP standard access list
100-199 IP extended access list
200-299 Protocol type-code access list
300-399 DECnet access list
400-499 XNS standard access list
500-599 XNS extended access list
600-699 Appletalk access list
700-799 48-bit MAC address access list
800-899 IPX standard access list
900-999 IPX extended access list
1000-1099 IPX SAP access list
1100-1199 Extended 48-bit MAC address access list
1200-1299 IPX summary address access list
1300-1999 IP standard access list (expanded range)
2000-2699 IP extended access list (expanded range
Remember, individual lines cannot be removed from a numbered access list.
The entire access list must be deleted and recreated. All new entries to a
numbered access list are added to the bottom.
Named access lists provide a bit more flexibility. Descriptive names can be
used to identify your access-lists. Additionally, individual lines can be
removed from a named access-list. However, like numbered lists, all new
entries are still added to the bottom of the access list.
There are two common types of named access lists:
• IP standard named access lists
• IP extended named access lists
Configuration of both numbered and named access-lists is covered later in
this section.
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200 Wild Card Masks
IP access-lists use wildcard masks to determine two things:
1. Which part of an address must match exactly
2. Which part of an address can match any number
This is as opposed to a subnet mask, which tells us what part of an address
is the network (subnet), and what part of an address is the host. Wildcard
masks look like inversed subnet masks.
Consider the following address and wildcard mask:
Address: 172.16.0.0
Wild Card Mask: 0.0.255.255
The above would match any address that begins “172.16.” The last two
octets could be anything. How do I know this?
Two Golden Rules of Access Lists:
1. If a bit is set to 0 in a wild-card mask, the corresponding bit in the
address must be matched exactly.
2. If a bit is set to 1 in a wild-card mask, the corresponding bit in the
address can match any number. In other words, we “don’t care”
what number it matches.
To see this more clearly, we’ll convert both the address and the wildcard
mask into binary: Address: 10101100.00010000.00000000.00000000
Wild Card Mask: 00000000.00000000.11111111.11111111
Any 0 bits in the wildcard mask, indicates that the corresponding bits in the
address must be matched exactly. Thus, looking at the above example, we
must exactly match the following in the first two octets:
10101100.00010000 = 172.16
Any 1 bits in the wildcard mask indicates that the corresponding bits can be
anything. Thus, the last two octets can be any number, and it will still match
this access-list entry.
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201
Wild Card Masks (continued)
If wanted to match a specific address with a wildcard mask (we’ll use an
example of 172.16.1.1), how would we do it?
Address: 172.16.1.1
Wild Card Mask: 0.0.0.0
Written out in binary, that looks like:
Address: 10101100.00010000.00000001.00000001
Wild Card Mask: 00000000.00000000.00000000.00000000
Remember what a wildcard mask is doing. A 0 indicates it must match
exactly, a 1 indicates it can match anything. The above wildcard mask has
all bits set to 0, which means we must match all four octets exactly.
There are actually two ways we can match a host:
• Using a wildcard mask with all bits set to 0 – 172.16.1.1 0.0.0.0
• Using the keyword “host” – host 172.16.1.1
How would we match all addresses with a wildcard mask?
Address: 0.0.0.0
Wild Card Mask: 255.255.255.255
Written out in binary, that looks like:
Address: 00000000.00000000.00000000.00000000
Wild Card Mask: 11111111.11111111.11111111.11111111
Notice that the above wildcard mask has all bits set to 1. Thus, each bit can
match anything – resulting in the above address and wildcard mask matching
all possible addresses.
There are actually two ways we can match all addresses:
• Using a wildcard mask with all bits set to 1 – 0.0.0.0 255.255.255.255
• Using the keyword “any” – any
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202
Standard IP Access List
access-list [1-99] [permit | deny] [source address] [wildcard mask] [log]
Standard IP access-lists are based upon the source host or network IP
address, and should be placed closest to the destination network.
Consider the following example:
In order to block network 172.18.0.0 from accessing the 172.16.0.0 network,
we would create the following access-list on Router A:
Router(config)# access-list 10 deny 172.18.0.0 0.0.255.255
Router(config)# access-list 10 permit any
Notice the wildcard mask of 0.0.255.255 on the first line. This will match
(deny) all hosts on the 172.18.x.x network.
The second line uses a keyword of any, which will match (permit) any other
address. Remember that you must have at least one permit statement in your
access list.
To apply this access list, we would configure the following on Router A:
Router(config)# int s0
Router(config-if)# ip access-group 10 in
To view all IP access lists configured on the router:
Router# show ip access-list
To view what interface an access-list is configured on:
Router# show ip interface
Router# show running-config
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203
Extended IP Access List
access-list [100-199] [permit | deny] [protocol] [source address] [wildcard
mask] [destination address] [wildcard mask] [operator [port]] [log]
Extended IP access-lists block based upon the source IP address, destination
IP address, and TCP or UDP port number. Extended access-lists should be
placed closest to the source network.
Consider the following example:
Assume there is a webserver on the 172.16.x.x network with an IP address
of 172.16.10.10. In order to block network 172.18.0.0 from accessing
anything on the 172.16.0.0 network, EXCEPT for the HTTP port on the web
server, we would create the following access-list on Router B:
Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80
Router(config)# access-list 101 deny ip 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255
Router(config)# access-list 101 permit ip any any
The first line allows the 172.18.x.x network access only to port 80 on the
web server. The second line blocks 172.18.x.x from accessing anything else
on the 172.16.x.x network. The third line allows 172.18.x.x access to
anything else.
We could have identified the web server in one of two ways:
Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80
Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 172.16.10.10 0.0.0.0 eq 80
To apply this access list, we would configure the following on Router B:
Router(config)# int e0
Router(config-if)# ip access-group 101 in
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204
Extended IP Access List Port Operators
In the preceding example, we identified TCP port 80 on a specific host use
the following syntax:
Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80
We accomplished this using an operator of eq, which is short for equals.
Thus, we are identifying host 172.16.10.10 with a port that equals 80.
We can use several other operators for port numbers:
eq Matches a specific port
gt Matches all ports greater than the port specified
lt Matches all ports less than the port specified
neq Matches all ports except for the port specified
range Match a specific inclusive range of ports
The following will match all ports greater than 100:
Router(config)# access-list 101 permit tcp any host 172.16.10.10 gt 100
The following will match all ports less than 1024:
Router(config)# access-list 101 permit tcp any host 172.16.10.10 lt 1024
The following will match all ports that do not equal 443:
Router(config)# access-list 101 permit tcp any host 172.16.10.10 neq 443
The following will match all ports between 80 and 88:
Router(config)# access-list 101 permit tcp any host 172.16.10.10 range 80 88
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205 Access List Logging
Consider again the following example:
Assume there is a webserver on the 172.16.x.x network with an IP address
of 172.16.10.10.
We wish to keep track of the number of packets permitted or denied by each
line of an access-list. Access-lists have a built-in logging mechanism for
such a purpose:
Router(config)# access-list 101 permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80 log
Router(config)# access-list 101 deny ip 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255 log
Router(config)# access-list 101 permit ip any any log
Notice we added an additional keyword log to each line of the access-list.
When viewing an access-list using the following command:
Router# show access-list 101
We will now have a counter on each line of the access-list, indicating the
number of packets that were permitted or denied by that line. This
information can be sent to a syslog server:
Router(config)# logging on Router(config)# logging 172.18.1.50
The logging on command enables logging. The second logging command
points to a syslog host at 172.18.1.50.
We can include more detailed logging information, including the source
MAC address of the packet, and what interface that packet was received on.
To accomplish this, use the log-input argument:
Router(config)# access-list 101 permit ip any any log-input
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
206 ICMP Access List
Consider this scenario. You’ve been asked to block anyone from the
172.18.x.x network from “pinging” anyone on the 172.16.x.x network. You
want to allow everything else, including all other ICMP packets.
The specific ICMP port that a “ping” uses is echo. To block specific ICMP
parameters, use an extended IP access list. On Router B, we would
configure:
Router(config)# access-list 102 deny icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255 echo
Router(config)# access-list 102 permit icmp 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255
Router(config)# access-list 102 permit ip any any
The first line blocks only ICMP echo requests (pings). The second line
allows all other ICMP traffic. The third line allows all other IP traffic.
Don’t forget to apply it to an interface on Router B:
Router(config)# int e0
Router(config-if)# ip access-group 102 in
Untrusted networks (such as the Internet) should usually be blocked from
pinging an outside router or any internal hosts:
Router(config)# access-list 102 deny icmp any any
Router(config)# access-list 102 permit ip any any
Router(config)# interface s0
Router(config-if)# ip access-group 102 in
The above access-list completed disables ICMP on the serial interface.
However, this would effectively disable ICMP traffic in both directions on
the router. Any replies to pings initiated by the Internal LAN would be
blocked on the way back in.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
207 Telnet Access List
We can create access lists to restrict telnet access to our router. For this
example, we’ll create an access list that prevents anyone from the evil
172.18.x.x network from telneting into Router A, but allow all other
networks telnet access.
First, we create the access-list on Router A:
Router(config)# access-list 50 deny 172.18.0.0 0.0.255.255
Router(config)# access-list 50 permit any
The first line blocks the 172.18.x.x network. The second line allows all other
networks.
To apply it to Router A’s telnet ports:
Router(config)# line vty 0 4
Router(config-line)# access-class 50 in
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
208 Named Access Lists
Named access lists provide us with two advantages over numbered access
lists. First, we can apply an identifiable name to an access list, for
documentation purposes. Second, we can remove individual lines in a named
access-list, which is not possible with numbered access lists.
Please note, though we can remove individual lines in a named access list,
we cannot insert individual lines into that named access list. New entries are
always placed at the bottom of a named access list.
To create a standard named access list, the syntax would be as follows:
Router(config)# ip access-list standard NAME
Router(config-std-nacl)# deny 172.18.0.0 0.0.255.255
Router(config-std-nacl)# permit any
To create an extended named access list, the syntax would be as follows:
Router(config)# ip access-list extended NAME
Router(config-ext-nacl)# permit tcp 172.18.0.0 0.0.255.255 host 172.16.10.10 eq 80
Router(config-ext-nacl)# deny ip 172.18.0.0 0.0.255.255 172.16.0.0 0.0.255.255
Router(config-ext-nacl)# permit ip any any
Notice that the actual configuration of the named access-list is performed in
a separate router “mode”:
Router(config-std-nacl)# Router(config-ext-nacl)#
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
209 Time-Based Access-Lists
Beginning with IOS version 12.0, access-lists can be based on the time and
the day of the week.
The first step to creating a time-based access-list, is to create a time-range:
Router(config)# time-range BLOCKHTTP
The above command creates a time-range named BLOCKHTTP. Next, we
must either specify an absolute time, or a periodic time:
Router(config)# time-range BLOCKHTTP
Router(config-time-range)# absolute start 08:00 23 May 2006 end 20:00 26 May 2006
Router(config)# time-range BLOCKHTTP
Router(config-time-range)# periodic weekdays 18:00 to 23:00
Notice the use of military time. The first time-range sets an absolute time
that will start from May 23, 2006 at 8:00 a.m., and will end on May 26,
2006 at 8:00 p.m.
The second time-range sets a periodic time that is always in effect on
weekdays from 6:00 p.m. to 11:00 p.m.
Only one absolute time statement is allowed per time-range, but multiple
periodic time statements are allowed.
After we establish our time-range, we must reference it in an access-list:
Router(config)# access-list 102 deny any any eq 80 time-range BLOCKHTTP
Router(config)# access-list 102 permit ip any any
Notice the time-range argument at the end of the access-list line. This will
result in HTTP traffic being blocked, but only during the time specified in
the time-range. Source: (http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t1/timerang.htm)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
210 Advanced Wildcard Masks
Earlier in this section, we discussed the basics of wildcard masks. The
examples given previously matched one of three things:
• A specific host
• A specific octet(s)
• All possible hosts
It is also possible to match groups or ranges of hosts with wildcard masks.
For example, assume we wanted a standard access-list that denied the
following hosts: 172.16.1.4 172.16.1.5 172.16.1.6 172.16.1.7
We could create an access-list with four separate lines:
Router(config)# access-list 10 deny 172.16.1.4 0.0.0.0
Router(config)# access-list 10 deny 172.16.1.5 0.0.0.0
Router(config)# access-list 10 deny 172.16.1.6 0.0.0.0
Router(config)# access-list 10 deny 172.16.1.7 0.0.0.0
However, it is also possible to match all four addresses in one line:
Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3
How do I know this is correct? Let’s write out the above four addresses, and
my wildcard mask in binary:
172.16.1.4: 10101100.00010000.00000001.00000100 172.16.1.5: 10101100.00010000.00000001.00000101 172.16.1.6: 10101100.00010000.00000001.00000110 172.16.1.7: 10101100.00010000.00000001.00000111
Wild Card Mask: 00000000.00000000.00000000.00000011
Notice that the first 30 bits of each of the four addresses are identical. Each
begin “10101100.00010000.00000001.000001”. Since those bits must match
exactly, the first 30 bits of our wildcard mask are set to 0.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
211
Advanced Wildcard Masks (continued)
Notice now that the only bits that are different between the four addresses
are the last two bits. Not only that, but we use every computation of those
last two bits: 00, 01, 10, 11.
Thus, since those last two bits can be anything, the last two bits of our
wildcard mask are set to 1.
The resulting access-list line:
Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3
We also could have determined the appropriate address and wildcard mask
by using AND/XOR logic.
To determine the address, we perform a logical AND operation:
1. If all bits in a column are set to 0, the corresponding address bit is 0
2. If all bits in a column are set to 1, the corresponding address bit is 1
3. If the bits in a column are a mix of 0’s and 1’s, the corresponding
address bit is a 0.
Observe: 172.16.1.4: 10101100.00010000.00000001.00000100 172.16.1.5: 10101100.00010000.00000001.00000101 172.16.1.6: 10101100.00010000.00000001.00000110 172.16.1.7: 10101100.00010000.00000001.00000111 Result: 10101100.00010000.00000001.00000100
Our resulting address is 172.16.1.4. This gets us half of what we need.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
212
Advanced Wildcard Masks (continued)
To determine the wildcard mask, we perform a logical XOR (exclusive OR)
operation:
1. If all bits in a column are set to 0, the corresponding wildcard bit is 0
2. If all bits in a column are set to 1, the corresponding wildcard bit is 0
3. If the bits in a column are a mix of 0’s and 1’s, the corresponding
wildcard bit is a 1.
Observe: 172.16.1.4: 10101100.00010000.00000001.00000100 172.16.1.5: 10101100.00010000.00000001.00000101 172.16.1.6: 10101100.00010000.00000001.00000110 172.16.1.7: 10101100.00010000.00000001.00000111 Result: 00000000.00000000.00000000.00000011
Our resulting wildcard mask is 0.0.0.3. Put together, we have:
Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3
Please Note: We can determine the number of addresses a wildcard mask
will match by using a simple formula:
2n
Where “n” is the number of bits set to 1 in the wildcard mask. In the above
example, we have two bits set to 1, which matches exactly four addresses
(22 = 4).
There will be occasions when we cannot match a range of addresses in one
line. For example, if we wanted to deny 172.16.1.4-6, instead of 172.16.1.4-
7, we would need two lines:
Router(config)# access-list 10 permit 172.16.1.7 0.0.0.0
Router(config)# access-list 10 deny 172.16.1.4 0.0.0.3
If we didn’t include the first line, the second line would have denied the
172.16.1.7 address. Always remember to use the above formula (2n) to
ensure your wildcard mask doesn’t match more addresses than you intended
(often called overlap).
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
213
Advanced Wildcard Masks (continued)
Two more examples. How would we deny all odd addresses on the
10.1.1.x/24 subnet in one access-list line?
Router(config)# access-list 10 deny 10.1.1.1 0.0.0.254
Written in binary: 10.1.1.1: 00001010.00000001.00000001.00000001
Wild Card Mask: 00000000.00000000.00000000.11111110
What would the result of the above wildcard mask be?
1. The first three octets must match exactly.
2. The last bit in the fourth octet must match exactly. Because we set this
bit to 1 in our address, every number this matches will be odd.
3. All other bits in the fourth octet can match any number.
Simple, right? How would we deny all even addresses on the 10.1.1.x/24
subnet in one access-list line?
Router(config)# access-list 10 deny 10.1.1.0 0.0.0.254
Written in binary: 10.1.1.0: 00001010.00000001.00000001.00000000
Wild Card Mask: 00000000.00000000.00000000.11111110
What would the result of the above wildcard mask be?
4. The first three octets must match exactly.
5. The last bit in the fourth octet must match exactly. Because we set this
bit to 0 in our address, every number this matches will be even.
6. All other bits in the fourth octet can match any number.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
214 Section 17
- Route Filtering and Route-Maps -
Prefix-Lists
Prefix-lists are used to match routes as opposed to traffic. Two things are
matched:
• The prefix (the network itself)
• The prefix-length (the length of the subnet mask)
Consider the following prefix-list:
Router(config)# ip prefix-list MYLIST 10.1.1.0/24
The above prefix-list matches the 10.1.1.0/24 network exactly. It will not
match 10.1.0.0/16, or 10.1.1.4/30.
A range of prefix-lengths can be specified:
Router(config)# ip prefix-list MYLIST 10.1.1.0/24 le 30
Router(config)# ip prefix-list MYLIST 10.1.1.0/24 ge 26 le 30
The first command dictates that the first 24 bits of the prefix must match
(meaning, the prefix must begin 10.1.1), and the subnet mask must be less
than or equal to 30 bits.
The second command dictates again that the first 24 bits of the prefix must
match, and the subnet mask must be between 26 to 30 bits (or equal to).
To match all prefixes:
Router(config)# ip prefix-list MYLIST 0.0.0.0/0 le 32
To view information about all prefix lists:
Router# show ip prefix-list detail
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
215 Distribute-Lists
Distribute-lists are used to filter routing updates, either inbound or
outbound. Routes must first be matched using an access-list or prefix-list,
and then applied using a distribute-list under the routing process:
To use an access-list to identify routes:
Router(config)# access-list 10 permit ip 172.16.0.0 0.0.255.255
Router(config)# router rip
Router(config-router)# distribute-list 10 in serial0/0
The above distribute-list will control routes sent inbound on serial0/0.
Specifically, the referenced access-list will only permit routes matching
172.16 in the first two octets.
To use a prefix-list to identify routes:
Router(config)# ip prefix-list MYLIST 10.1.0.0/16
Router(config)# router rip
Router(config-router)# distribute-list prefix MYLIST out fastethernet0/0
The above distribute-list will control routes sent outbound on
fastethernet0/0. Specifically, the referenced prefix-list will only match the
exact 10.1.0.0/16 route.
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
216 Route-Maps
Route-maps are advanced access-lists that serve several functions on IOS
devices, including (but not limited to):
• Controlling redistribution between routing protocols.
• Adjusting the attributes of routes (especially for BGP).
• Implementing Policy Based Routing (PBR).
As with access-lists, route-maps are organized as a sequential set of rules or
statements, each with a permit or deny condition. However, access-lists
can merely permit or deny traffic, while a route-map can additionally modify
or perform a specific action on traffic.
Route-maps follow a very simple logic:
• Traffic must be first matched, based on specified criteria.
• A particular attribute or action is set on the matched traffic.
Each statement in a route-map is assigned a sequence number, and contains
a series of match and set statements. The route-map is parsed from the
lowest sequence number to the highest, and will stop once a match is found.
The following demonstrates the syntax of a route-map:
Router(config)# access-list 1 permit 10.1.1.0 0.0.0.255
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set ip next-hop 192.168.1.1
First, an access-list was created that matched traffic from 10.1.1.0/24.
Then, a route-map called MYMAP was created, and assigned a sequence
number of 10 with a permit condition. If a route-map contains multiple
statements, the sequence number dictates the order of those statements.
The route-map will then match any traffic listed in access-list 1. Notice that
the syntax to call an access-list match ip address.
Finally, the desired attributed is set to this traffic. In this instance, the ip next
hop attribute has been modified to 192.168.1.1.
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
217 Route-Maps (continued)
A single route-map statement can contain multiple match commands:
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1 2 3
The above line would match traffic in access-list 1, or access-list 2, or
access-list 3. Thus, when match criteria is contained within a single line, a
logical OR is applied.
However, if match criteria is specified on separate lines:
Router(config-route-map)# match ip address 1
Router(config-route-map)# match ip address 2
Then the traffic must match access-list 1 and access-list 2 (a logical AND).
Remember this distinction!
If no match criteria is specified, all traffic is matched!
Additionally, a single route-map statement can contain multiple set
commands:
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set weight 50
Router(config-route-map)# set local-preference 200
Any traffic matching access-list 1 will have both set attributes applied.
There is an implicit deny any statement at the bottom of every route-map.
The impact of this deny any statement is dependent on the function of the
access-list:
• If using a route-map for policy-based routing or adjusting
attributes, any routes/traffic not specifically matched will remain
unchanged.
• If using a route-map for redistribution, any routes not specifically
matched (and permitted) will not be redistributed.
(Reference: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008047915d.shtml)
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
218 Route-Map Criteria
The following are example attributes that can be matched by a route-map:
• match ip address
• match interface
• match ip address prefix-list
• match ip next-hop
• match metric • match route-type • match tag • match community
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# match interface serial0/0
Router(config-route-map)# match ip address prefix-list MYLIST
Router(config-route-map)# match ip next-hop 192.168.1.2
Router(config-route-map)# match metric 40
Router(config-route-map)# match route-type internal
Router(config-route-map)# match tag 33
Router(config-route-map)# match community 123
The following are example attributes that can be set by a route-map:
• set interface
• set ip next-hop
• set metric • set tag • set community • set local-preference • set weight
• set ip precedence
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# set interface fastethernet0/1
Router(config-route-map)# set ip next-hop 10.1.1.1
Router(config-route-map)# set metric 200
Router(config-route-map)# set tag 44
Router(config-route-map)# set community 321
Router(config-route-map)# set local-preference 250
Router(config-route-map)# set weight 300
Router(config-route-map)# set ip precedence 2
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This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
219 Route-Map Examples
The following route-map is applying a BGP attribute to a specific route:
Router(config)# access-list 1 permit 10.1.1.0 0.0.0.255
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set metric 100
Router(config-route-map)# route-map MYMAP permit 20
Router(config)# router bgp 100
Router(config-router)# neighbor 172.16.1.1 route-map MYMAP out
The following route-map is controlling routes being redistributed between
routing protocols:
Router(config)# access-list 1 deny 192.168.1.0 0.0.255
Router(config)# access-list 1 deny 192.168.2.0 0.0.255
Router(config)# access-list 1 permit any
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set tag 150
Router(config)# router ospf 1
Router(config-router)# redistribute eigrp 10 metric 3 subnets route-map MYMAP
The following route-map is manipulating inbound traffic on a specific
interface:
Router(config)# access-list 1 permit 10.1.1.0 0.0.0.255
Router(config)# route-map MYMAP permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set ip next-hop 192.168.1.1
Router(config)# interface s0/0
Router(config-if)# ip policy route-map MYMAP
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
220 Section 18 - Multicast - Types of “packets”
Three types of packets can exist on an IPv4 network:
Unicast – A packet sent from one host to only one other host. A hub will
forward a unicast out all ports. If a switch has a table entry for the unicast’s
MAC address, it will forward it out only the appropriate port.
Broadcast – A packet sent from one host to all hosts on the IP subnet. Both
hubs and switches will forward a broadcast out all ports. By definition, a
router will not forward a broadcast from one segment to another.
Multicast – A packet sent from one host to a specific group of hosts.
Switches, by default, will forward a multicast out all ports. A router, by
default, will not forward a multicast from one segment to another.
Multicast Concepts
Remember, a multicast is a packet sent from one computer to a group of
hosts. A host must join a multicast group in order to accept a multicast.
Joining a multicast group can be accomplished statically or dynamically.
Multicast traffic is generally sent from a multicast server, to multicast
clients. Very rarely is a multicast packet sent back from a client to the
server.
Multicasts are utilized in a wide range of applications, most notably voice or
video systems that have one source “serving” out data to a very specific
group of clients.
The key to configuring multicast is to ensure only the hosts that require the
multicast traffic actually receive it.
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
221 Multicast Addressing
IPv4 addresses are separated into several “classes.”
Class A: 1.1.1.1 – 127.255.255.255
Class B: 128.0.0.0 – 191.255.255.255
Class C: 192.0.0.0 – 223.255.255.255
Class D: 224.0.0.0 – 239.255.255.255
Class D addresses have been reserved for multicast. Within the Class D
address space, several ranges have been reserved for specific purposes:
• 224.0.0.0 – 224.0.0.255 – Reserved for routing and other network
protocols, such as OSPF, RIP, VRRP, etc.
• 224.0.1.0 – 238.255.255.255 – Reserved for “public” use, can be used
publicly on the Internet. Many addresses in this range have been
reserved for specific applications
• 239.0.0.0 – 239.255.255.255 – Reserved for “private” use, and cannot
be routed on the Internet.
The following outlines several of the most common multicast addresses
reserved for routing protocols:
• 224.0.0.1 – all hosts on this subnet
• 224.0.0.2 – all routers on this subnet
• 224.0.0.5 – all OSPF routers
• 224.0.0.6 – all OSPF Designated routers
• 224.0.0.9 – all RIPv2 routers
• 224.0.0.10 – all IGRP routers
• 224.0.0.12 – DHCP traffic
• 224.0.0.13 – all PIM routers
• 224.0.0.19-21 – ISIS routers
• 224.0.0.22 – IGMP traffic
• 224.0.1.39 – Cisco RP Announce
• 224.0.1.40 – Cisco RP Discovery
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
222 Multicast MAC Addresses
Unfortunately, there is no ARP equivalent protocol for multicast addressing.
Instead, a reserved range of MAC addresses were created for multicast IPs.
All multicast MAC addresses begin with:
0100.5e
Recall that the first six digits of a MAC address identify the vendor code,
and the last 6 digits identify the specific host address. To complete the MAC
address, the last 23 bits of the multicast IP address are used.
For example, consider the following multicast IP address and its binary
equivalent: 224.65.130.195 = 11100000.01000001.10000010.11000011
Remember that a MAC address is 48 bits long, and that a multicast MAC
must begin with 0100.5e. In binary, that looks like:
00000001.00000000.01011110.0
Add the last 23 bits of the multicast IP address to the MAC, and we get:
00000001.00000000.01011110.01000001.10000010.11000011
That should be exactly 48 bits long. Converting that to Hex format, our full
MAC address would be:
0100.5e41.82c3
How did I convert this to Hex? Remember that hexadecimal is Base 16
mathematics. Thus, to represent a single hexadecimal digit in binary, we
would need 4 bits (24 = 16). So, we can break down the above binary MAC
address into groups of four bits:
Binary 0000 0001 0000 0000 0101 1110 0100 0001 1000 0010 1100 0011
Decimal 0 1 0 0 5 14 4 1 8 2 12 3
Hex 0 1 0 0 5 e 4 1 8 2 c 3
Hence the MAC address of 0100.5e41.82c3.
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unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
223
Multicast MAC Addresses (continued)
Ready for some more math, you binary fiends?
Calculate what the multicast MAC address would be for the following IP
addresses: 225.2.100.15 = 11100001.00000010.01100100.00001111 231.130.100.15 = 11100111.10000010.01100100.00001111
Remember that all multicast MACs begin with:
0100.5e = 00000001.00000000.01011110.0
So, add the last 23 digits of each of the above IP addresses to the MAC
address, and we get:
225.2.100.15 = 00000001.00000000.01011110.00000010.01100100.00001111 231.130.100.15 = 00000001.00000000.01011110.00000010.01100100.00001111
In Hex, that would be:
225.2.100.15 = 0100.5e02.640f 231.130.100.15 = 0100.5e02.640f
Wait a second…. That’s the exact same multicast MAC address, right?
Double-checking our math, we see that it’s perfect.
Believe it or not, each multicast MAC address can match 32 multicast IP
addresses, because we’re only taking the last 23 bits of our IP address.
We already know that all multicast IP addresses MUST begin 1110. Looking
at the 225.2.100.15 address in binary:
11100001.00000010.01100100.00001111
That leaves 5 bits in between our starting 1110, and the last 23 bits of our IP.
Those 5 bits could be anything, and the multicast MAC address would be the
same. Because 25 = 32, there are 32 multicast IP’s per multicast MAC.
According to the powers that be, the likelihood of two multicast systems
utilizing the same multicast MAC is rare. The worst outcome would be that
hosts joined to either multicast system would receive multicasts from both.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
224 Multicasts and Routing
A router, by default, will drop multicast traffic, unless a Multicast routing
protocol is utilized. Multicast routing protocols ensure that data sent from a
multicast source are received by (and only by) its corresponding multicast
clients.
Several multicast routing protocols exist, including:
• Protocol Independent Multicast (PIM)
• Multicast OSPF (MOSPF)
• Distance Vector Multicast Routing Protocol (DVMRP)
• Core-Based Trees (CBT)
Multicast routing must be enabled globally on a Cisco router or switch,
before it can be used:
Switch(config)# ip multicast-routing Multicast Path Forwarding
Normally, routers build routing tables that contain destination addresses,
and route packets towards that destination. With multicast, routers are
concerned with routing packets away from the multicast source. This
concept is called Reverse Path Forwarding (RPF).
Multicast routing protocols build tables that contain several elements:
• The multicast source, and its associated multicast address (labeled as
“S,G”, or “Source,Group”)
• Upstream interfaces that point towards the source
• Downstream interfaces that point away from the source towards
multicast hosts.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
225
Multicast Path Forwarding Example
A router interface will not be designated as a downstream interface unless
multicast hosts actually exist downstream. In the above example, no
multicast hosts exist downstream of Router 5.
In fact, because no multicast hosts exist downstream of Router 1 towards
Router 2, no multicast traffic for this multicast group will be forwarded
down that path. Thus, Router 1’s interface connecting to Router 2 will not
become a downstream port.
This pruning allows for efficient use of bandwidth. No unnecessary traffic is
sent down a particular link. This “map” of which segments contain multicast
hosts is called the multicast tree. The multicast tree is dynamically updated
as hosts join or leave the multicast group (otherwise known as pruning the
branches).
By designating upstream and downstream interfaces, the multicast tree
remains loop-free. No multicast traffic should ever be sent back upstream
towards the multicast source.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
226
Internet Group Management Protocol (IGMP)
Remember, multicast works by having a source send data to a specific set of
clients that belong to the same multicast group. The multicast group is
configured (or assigned) a specific multicast address.
The multicast clients need a mechanism to join multicast groups. Internet
Group Management Protocol (IGMP) allows clients to send “requests” to
multicast-enabled routers to join a multicast group.
IGMP only handles group membership. To actually route multicast data to a
client, a multicast routing protocol is required, such as PIM or DVMRP.
Three versions of IGMP exist, IGMPv1, IGMPv2, and IGMPv3.
IGMPv1 routers send out a “query” every 60 seconds to determine if any
hosts need access to a multicast server. This query is sent out to the
224.0.0.1 address (i.e., all hosts on the subnet). Interested hosts must reply
with a Membership Report stating what multicast group they wish to join.
Unfortunately, IGMPv1 does not allow hosts to dynamically “leave” a
group. Instead, if no Membership Reports are received after 3 times the
query interval, the router will flush the hosts out of its IGMP table.
IGMPv2 adds additional functionality. Queries can be sent out either as
General Queries (224.0.0.1) or Group-Specific Queries (only sent to
specific group members). Additionally, hosts can send a Leave Group
message to IGMPv2 routers, to immediately be flushed out of the IGMP
table. Thus, IGMPv2 allows the multicast tree to by updated more
efficiently.
All versions of IGMP elect one router to be the Designated Querier for that
subnet. The router with the lowest IP address becomes Designated.
IGMPv1 is not compatible with IGMPv2. If any IGMPv1 routers exist on
the network, all routers must operate in IGMPv1 mode.
Cisco IOS version 11.1 and later support IGMPv2 by default.
IGMPv3 enhances v2 by supporting source-based filtering of multicast
groups. Essentially, when a host responds to an IGMP query with a
Membership Report, it can specifically identify which sources within a
multicast group to join (or even not join).
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
227 IGMP Example
In the above example, assume the router is using IGMPv2. Interface fa0/1
points towards the multicast source, and thus becomes the upstream
interface.
Initially, the router will sent out Group Specific Queries out all nonupstream
interfaces. Any multicast hosts will respond with a Membership
Report stating what multicast group they wish to join.
Interfaces fa0/2 and fa0/3 will become downstream interfaces, as they
contain multicast hosts. No multicast traffic will be sent out fa0/4.
If all multicast hosts leave the multicast group off of interface fa0/2, it will
be removed from the multicast tree. If a multicast host is ever added off of
interface fa0/4, it will become a downstream interface.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
228 IGMP Configuration
No configuration is required to enable IGMP, except to enable IP multicast
routing (ip multicast-routing). We can change the version of IGMP running
on a particular interface (by default, it is Version 2):
Switch(config-if)# ip igmp version 1
To view which multicast groups the router is aware of:
Switch# show ip igmp groups
We can join a router interface to a specific multicast group (forcing the
router to respond to ICMP requests to this multicast group):
Switch(config-if)# ip igmp join-group 226.1.5.10
WE can also simply force a router interface to always forward the traffic of a
specific multicast group out an interface:
Switch(config-if)# ip igmp static-group 226.1.5.10
We can also restrict which multicast groups a host, off of a particular
interface, can join:
Switch(config)# access-list 10 permit 226.1.5.10
Switch(config)# access-list 10 permit 226.1.5.11
Switch(config-if)# ip igmp access-group 10
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
229
Protocol Independent Multicast (PIM)
While IGMP concerns itself with allowing multicast hosts to join multicast
groups, Protocol Independent Multicast (PIM) is a multicast routing
protocol that is concerned about getting the multicast data to its destination
(or, more accurately, taking the data away from the multicast source).
PIM is also responsible for creating the multicast tree, and “pruning” the tree
so that no traffic is sent unnecessarily down a link.
PIM can operate in three separate modes:
• PIM Dense Mode (PIM-DM)
• PIM Sparse Mode (PIM-SM)
• PIM Sparse-Dense Mode (PIM-SM-DM, Cisco proprietary)
The key difference between PIM Dense and Sparse Mode is how the
multicast tree is created. With PIM Dense Mode, all networks are flooded
with the multicast traffic from the source. Afterwards, networks that don’t
need the multicast are pruned off of the tree. The network that contains the
multicast source becomes the “root” of the multicast network.
With PIM Sparse Mode, no “flooding” occurs. Only networks that contain
“requesting” multicast hosts are added to the multicast tree. A centralized
PM router, called the Rendezvous Point (RP), is elected to be the “root”
router of the multicast tree. PIM routers operating in Sparse Mode build their
tree towards the RP, instead of towards the multicast source. The RP allows
multiple multicast “sources” to utilize the same multicast tree.
PIM Sparse-Dense Mode allows either Sparse or Dense Mode to be used,
depending on the multicast group. Any group that points to an RP utilizes
Sparse Mode. PIM Sparse-Dense Mode is Cisco proprietary.
Consider these key points:
• Dense Mode should be used when a large number of multicast hosts
exist across the internetwork. The “flooding” process allows for a
quick creation of the multicast tree, at the expense of wasting
bandwidth.
• Sparse Mode should be used when only a limited number of
multicast hosts exist. Because hosts must explicitly join before that
network segment is added to the multicast tree, bandwidth is utilized
more efficiently.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
230
PIM Dense Mode Example
Multicast Source
Multicast Hosts No Multicast Multicast Hosts
No Multicast Hosts Hosts Router 1
Router 5 Router 6
Router 2 Router 3 Router 4
Router 7
Consider the above example. When PIM routers operate in Dense Mode, all
segments of the multicast tree are flooded initially. Eventually, “branches”
that do not require the multicast traffic are pruned off:
Multicast Source
Multicast Hosts No Multicast Multicast Hosts
No Multicast Hosts Hosts Router 1
Router 5 Router 6
Router 2 Router 3 Router 4
Router 7
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
231
PIM Sparse Mode Example
When PIM routers operate in Sparse Mode, multicast traffic is not initially
flooded throughout the entire multicast tree. Instead, a Rendezvous Point
(RP) is elected or designated, and all multicast sources and clients must
explicitly register with the RP. This provides a centralized method of
directing the multicast traffic of multiple multicast sources:
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
232 Configuring Manual PIMv1
Two versions of PIM exist (PIMv1 and PIMv2), though both are very
similar. PIM must be enabled on each participating interface in the multicast
tree.
To enable PIM and specify its mode on an interface:
Switch(config)# interface fa0/10 Switch(config-if)# no switchport
Switch(config-if)# ip pim dense-mode
Switch(config-if)# ip pim sparse-mode
Switch(config-if)# ip pim sparse-dense-mode
When utilizing PIM-SM, we must configure a Rendezvous Point (RP). RP’s
can be identified manually, or dynamically chosen using a process called
auto-RP (Cisco-proprietary).
To manually specify an RP on a router:
Switch(config)# ip pim rp-address 192.168.1.1
The above command must be configured on every router in the multicast
tree, including the RP itself.
To restrict the RP to a specific set of multicast groups:
Switch(config)# access-list 10 permit 226.10.10.1
Switch(config)# access-list 10 permit 226.10.10.2
Switch(config)# ip pim rp-address 192.168.1.1 10
The first two commands create an access-list 10 specifying the multicast
groups this RP will support. The third command identifies the RP, and
applies access-list 10 to the RP.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
233 Configuring Dynamic PIMv1
When using Cisco’s auto-RP, one router is designated as a Mapping Agent.
To configure a router as a mapping agent:
Switch(config)# ip pim send-rp-discovery scope 10
The 10 parameter in the above command is a TTL (Time to Live) setting,
indicating that this router will serve as a mapping agent for up to 10 hops
away.
Mapping agents listen for candidate RP’s over multicast address 224.0.1.39
(Cisco RP Announce). To configure a router as a candidate RP:
Switch(config)# access-list 10 permit 226.10.10.1
Switch(config)# access-list 10 permit 226.10.10.2
Switch(config)# ip pim send-rp-announce fa0/10 scope 4 group-list 10
The first two commands create an access-list 10 specifying the multicast
groups this RP will support. The third command identifies this router as a
candidate RP for the multicast groups specified in group-list 10. This RP’s
address will be based on the IP address configured on fa0/10. The scope 4
parameter indicates the maximum number of hops this router will advertise
itself for.
The above commands essentially create a “mapping” of specific RP’s to
specific multicast groups. Once a mapping agent learns of these mappings
from candidate RPs, it sends the information to all PIM routers over
multicast address 224.0.1.40 (Cisco RP Discovery).
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
234 Configuring Dynamic PIMv2
Configuring PIMv2 is very similar to PIMv1, except that PIMv2 is a
standards-based protocol. Also, there are terminology differences. Instead of
mapping agents, PIMv2 uses Bootstrap Routers (BSR), which performs the
same function.
To configure a router as a BSR:
Switch(config)# ip pim bsr-candidate fa0/10
To configure candidate RP’s in PIMv2:
Switch(config)# access-list 10 permit 226.10.10.1
Switch(config)# access-list 10 permit 226.10.10.2
Switch(config)# ip pim rp-candidate fa0/10 4 group-list 10
The first two commands create an access-list 10 specifying the multicast
groups this RP will support. The third command identifies this router as a
candidate RP for the multicast groups specified in group-list 10. This RP’s
address will be based on the IP address configured on fa0/10. The 4
parameter indicates the maximum number of hops this router will advertise
itself for.
With PIMv2, we can create border routers to prevent PIM advertisements
(from the BSR or Candidate RPs) from passing a specific point.
To configure a router as a PIM border router:
Switch(config)# ip pim border
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
235
Multicasts and Layer 2 Switches
Up to this point, we’ve discussed how multicasts interact with routers or
multilayer switches.
By default, a Layer 2 switch will forward a multicast out all ports, excluding
the port it received the multicast on. To eliminate the need of “flooding”
multicast traffic, two mechanisms have been developed for Layer 2
switches: • IGMP snooping • CGMP
IGMP snooping allows a Layer 2 switch to “learn” the multicast MAC
address of multicast groups. It does this by eavesdropping on IGMP
Membership Reports sent from multicast hosts to PIM routers. The Layer 2
switch then adds a multicast MAC entry in the CAM for the specific port
that needs the multicast traffic.
IGMP snooping is enabled by default on the Catalyst 2950 and 3550. If
disabled, it can be enabled with the following command:
Switch(config)# ip igmp snooping
If a Layer 2 switch does not support IGMP snooping, Cisco Group
Membership Protocol (CGMP) can be used. Three guesses as to whether
this is Cisco-proprietary or not.
Instead of the Layer 2 switch “snooping” the IGMP Membership Reports,
CGMP allows the PIM router to actually inform the Layer 2 switch of the
multicast MAC address, and the MAC of the host joining the group. The
Layer 2 switch can then add this information to the CAM.
CGMP must be configured on the PIM router (or multilayer switch). It is
disabled by default on all PIM routers. To enable CGMP:
Switch(config-if)# ip cgmp
No configuration needs to occur on the Layer 2 switch.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
236 Troubleshooting Multicasting
To view IGMP groups and current members:
Switch# show ip igmp groups
To view the IGMP snooping status:
Switch# show ip igmp snooping
To view PIM “neighbors”:
Switch# show ip pim neighbor
To view PIM RPs:
Switch# show ip pim rp
To view PIM RP-to-Group mappings:
Switch# show ip pim rp mapping
To view the status of PIMv1 Auto-RP:
Switch# show ip pim autorp
To view PIMv2 BSRs:
Switch# show ip pim bsr-router
We can also debug multicasting protocols:
Switch# debug ip igmp
Switch# debug ip pim
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
237
Viewing the Multicast Table
Just like unicast routing protocols (such as OSPF, RIP), multicast routing
protocols build a routing table.
Again, these tables contain several elements:
• The multicast source, and its associated multicast address (labeled as
“S,G”, or “Source,Group”)
• Upstream interfaces that point towards the source
• Downstream interfaces that point away from the source towards
multicast hosts.
To view the multicast routing table:
Switch# show ip mroute
If using PIM in Dense Mode, the output would be similar to the following:
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned
R - RP-bit set, F - Register flag, T - SPT-bit set
Timers: Uptime/Expires
Interface state: Interface, Next-Hop, State/Mode
(10.1.1.1/24, 239.5.222.1), uptime 1:11:11, expires 0:04:29, flags: C
Incoming interface: Serial0, RPF neighbor 10.5.11.1
Outgoing interface list: Ethernet0, Forward/Sparse, 2:52:11/0:01:12
Remember that a multicast source with its associated multicast address is
labeled as (S,G). Thus, in the above example, 10.1.1.1/24 is the multicast
source, while 239.5.222.1 is the multicast address/group that the source
belongs to.
The Incoming interface indicates the upstream interface. The RPF neighbor
is the next hop router “upstream” towards the source. The outgoing
interface(s) indicate downstream interfaces.
Notice that the S – Sparse flag is not set. That’s because PIM is running in
Dense Mode.
CCNP Routing Study Guide v1.12 – Aaron Balchunas
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All original material copyright © 2007 by Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
238
Viewing the Multicast Table (continued)
Remember, to view the multicast routing table:
Switch# show ip mroute
If using PIM in Sparse Mode, the output would be similar to the following:
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned
R - RP-bit set, F - Register flag, T - SPT-bit set
Timers: Uptime/Expires
Interface state: Interface, Next-Hop, State/Mode
(*, 224.59.222.10), uptime 2:11:05, RP is 10.1.1.10, flags: SC
Incoming interface: Serial0, RPF neighbor 10.3.35.1,
Outgoing interface list: Ethernet0, Forward/Sparse, 4:41:22/0:05:21
Notice that the (S,G) pairing is labeled as (*, 224.59.222.10). In Sparse
Mode, we can have multiple sources share the same multicast tree.
The Rendezvous Point (RP) is 10.1.1.10. The flags are set to SC, indicating
this router is running in Sparse Mode.
Just like with Dense Mode, the Incoming interface indicates the upstream
interface, and the outgoing interface(s) indicate downstream interfaces.
However, the RPF neighbor is the next hop router “upstream” towards the
RP now, and not the source.
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