WhyRip IGRP Discontiguous.pdf

Cisco − Why Don't RIPv1 and IGRP Support Discontiguous Networks?

Table of Contents

Why Don't RIPv1 and IGRP Support Discontiguous Networks?. .................................................................1

Document ID: 13724.. ..............................................................................................................................1

Introduction. .........................................................................................................................................................1

Prerequisites. ........................................................................................................................................................1

Requirements. .........................................................................................................................................1

Components Used. ..................................................................................................................................1

Conventions. ...........................................................................................................................................2

Background Information. .....................................................................................................................................2

Sending Routing Updates. ...................................................................................................................................2

Receiving Routing Updates .................................................................................................................................2

Solution. ...............................................................................................................................................................3

Establishing Connectivity. ......................................................................................................................3

NetPro Discussion Forums − Featured Conversations. .......................................................................................3

Related Information. ............................................................................................................................................4

Why Don't RIPv1 and IGRP Support Discontiguous

Networks?

Document ID: 13724

Introduction

Prerequisites

Requirements

Components Used

Conventions

Background Information

Sending Routing Updates

Receiving Routing Updates

Solution

Establishing Connectivity

NetPro Discussion Forums − Featured Conversations

Related Information

Introduction

A discontiguous network is comprised of a major net separated by another major net. In the figure below,

network 131.108.0.0 is separated by a subnet of network 137.99.0.0; 131.108.0.0 is a discontiguous network

This document will describe why RIPv1 and IGRP do not support discontiguous networks and how to work

around it.

Prerequisites

Requirements

Readers of this document should have knowledge of these topics:

· IP Addressing and Subnetting

Configuring RIPv1 and IGRP

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment setup in a

topology very similar to the one shown above. All of the devices used in this document started with a cleared

(default) configuration. If your network is live, make sure that you understand the potential impact of any

Cisco − Why Don't RIPv1 and IGRP Support Discontiguous Networks?

command.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Background Information

RIP and IGRP are classful protocols. Whenever RIP advertises a network across a different major net

boundary, RIP summarizes the advertised network at the major net boundary. In the figure above, when

Router 1 sends an update containing 131.108.5.0 to Router 2 across 137.99.88.0, it converts 131.108.5.0/24

into 131.108.0.0/16. This process is called summarization.

Sending Routing Updates

Using the topology shown above, let's follow the steps and see what questions need to be answered when

Router 1 prepares to send an update to Router 2. More detailed information about this decision−making is

given in Behavior of RIP and IGRP When Sending and Receiving Updates. Keep in mind the network we are

interested in at this point is the advertisement of network 131.108.5.0/24.

Is 131.108.5.0/24 part of the same major net as 137.99.88.0/24, which is the network assigned to the

interface that's sourcing the update?

No: Router 1 summarizes 131.108.5.0/24 and advertises the route 131.108.0.0/16. The

summarization is done to the major classful boundary. In this case since it's a class B address,

the summary is 16 bits.

Yes: Although this is not the case in our example, if the answer to the above question was

yes, then Router 1 would not summarize the network and would advertise the network with

subnet information intact.

Using the debug ip rip command on Router 1, we can see the update sent by Router 1 in the output below:

RIP: sending v1 update to 255.255.255.255 via Serial3/0 (137.99.88.2)

RIP: build update entries

network 131.108.0.0 metric 1

Receiving Routing Updates

Now let's follow see what questions need to be answered when Router 2 prepares to receive and update from

Router 1. Again keep in mind the network we are interested in at this point is the reception of network

131.108.5.0/24. However, remember that when Router 1 sent the update the network was summarized to

131.108.0.0/16

Is the network being received (131.108.0.0/16) part of the same major network of 137.199.88.0,

which is the address assigned to the interface that received the update?

No: Do any subnets of this major network already exist in the routing table known from

interfaces other than that which received the update?

Yes: Ignore the update.

Again, using the debug ip rip command Router 2, we can see the update received from Router 1:

Cisco − Why Don't RIPv1 and IGRP Support Discontiguous Networks?

RIP: received v1 update from 137.99.88.2 on Serial2/0

131.108.0.0 in 1 hops

However, displaying the routing table of Router 2, we see that the update was ignored. The only entry for any

subnetwork or network on 131.108.0.0 is the one directly connected to Ethernet0. The output of the show ip

route command on Router 2 shows:

137.99.0.0/24 is subnetted, 1 subnets

C 137.99.88.0 is directly connected, Serial2/0

131.108.0.0/24 is subnetted, 1 subnets

C 131.108.2.0 is directly connected, Ethernet0/0

The behavior of RIPv1 and IGRP when sending and receiving routing updates results in Router 1 and Router

2 not learning about the attached subnetworks of 131.108.5.0/24 and 131.108.2.0/24. Because of this devices

on these two subnetworks would not be able to communicate with each other.

Solution

There may be some situations where discontiguous networks are unavoidable. In these situations it is

recommended that RIPv1 or IGRP not be used. Routing protocols such as EIGRP or OSPF are better suited

for this situation.

Establishing Connectivity

In the event that RIPv1 or IGRP is used with discontiguous networks, then static routes must be used to

establish connectivity between the discontiguous subnetworks. In this example the following static routes

would establish this connectivity:

For Router 1:

ip route 131.108.2.0 255.255.255.0 137.99.88.1

For Router 2:

ip route 131.108.5.0 255.255.255.0 137.99.88.2

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Cisco − Why Don't RIPv1 and IGRP Support Discontiguous Networks?

Related Information

· IP Routing Support Page

IP Routed Protocols Support Page

· IGRP Support Page

· RIP Support Page

· Technical Support − Cisco Systems

Updated: Feb 16, 2005

Document ID: 13724

Cisco − Why Don't RIPv1 and IGRP Support Discontiguous Networks?

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