Behavior of RIP and IGRP When Sending and Receiving UpdatesPrintable Pdf
Document ID: 13723.
Introduction Prerequisites
Requirements
Components Used
Conventions General Behavior
Send Updates
Receive Updates Specific Case
Send Updates
Receive Updates NetPro Discussion Forums - Featured Conversations Related Information
Introduction
This document explains the series of actions taken by both Routing Information Protocol (RIP) and Interior
Gateway Routing Protocol (IGRP) when they send or receive the routing updates.
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
The information in this document applies to these software and hardware versions:
Cisco IOS Software Release 12.2(27)
Cisco 2500 Series Routers
The information in this document was created from the devices in a specific lab environment. 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 command.
Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
General Behavior
Send Updates
When RIP or IGRP send an update, they perform certain checks before they advertise the update. This list
shows the sequence of events that occurs before Router 1 sends updates to Router 2. The network diagram
allows you to examine the sequence of events more closely.
Is the subnet information part of the same major net as the interface that sources the update?
No: Router 1 summarizes at the major net boundary and advertises the network.
Yes: Does the network have the same subnet mask as the interface that sources the update?
Yes: Router 1 advertises the subnet.
No: Does the network have a /32 mask
Yes:If it is RIP, then the network is advertised. If it is IGRP, then Router 1
drops the network.
No: Router 1 drops the network.
Receive Updates
When RIP or IGRP receive an update, they perform certain checks before they accept the update and apply
the subnet mask. This is the sequence of events that occurs before Router 2 accepts an update from Router 1:
Is the subnet received in the update on the same major net as the interface that received the update?
Yes: Router 2 applies the mask of the interface that received the update. If the advertised
network has a host bit set in the host portion of the update, Router 2 applies the host mask
(/32). In the case of RIP, it continues to advertise the /32 route to the subsequent router, but
IGRP does not.
No: Do any subnets of this major net already exist in the routing table, known from interfaces
other than the one that received the update? The network in this update should be a major net
unless the link between the two routers is an unnumbered link, in which case it is possible for
the update to contain subnet information.
Yes: Router 2 ignores the update.
No: Router 2 applies a classful mask. If the update came across an unnumbered link
and contains subnet information (bits in subnet portion of network are set), then
Router 2 applies a host mask. Refer to Understanding and Configuring the ip
unnumbered Command for unnumbered case examples.
Specific Case
Send Updates
When Router 1 sends an update to Router 2, it performs these checks:
Is 131.108.5.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
Yes: Does 131.108.5.0/24 have the same subnet mask as 131.108.2.0/24, which sources the
update?
Yes: Router 1 advertises the network.
Is 137.99.88.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
No: Router 1 summarizes 137.99.88.0/24 at the major net boundary and advertises the route
as 137.99.0.0.
This process results in Router 1 including 131.108.5.0 and 137.99.0.0 in its update to Router 2. You can see
this in the debug ip rip command output shown on Router 1:
When you issue the debug ip rip command, you can see the routing update received on Router 2 from Router
1:
*Mar 25 00:22:46.201: RIP: received v1 update from 131.108.2.2 on Serial0
*Mar 25 00:22:46.203:131.108.5.0 in 1 hops
*Mar 25 00:22:46.205:137.99.0.0 in 1 hops
Look at the checks Router 2 performs in order to determine what mask to apply on a received network.
Is the received major net 137.99.0.0 the same as 131.108.2.0, which is the address assigned to the
interface that received the update?
No: Do any subnets of this major net already exist in the routing table known from other
interfaces?
No: Router 2 applies the natural mask (/16) because 137.99.0.0 is a class B address.
Does subnet 131.108.5.0 belong to the same major net as subnet 131.108.2.0, which is the interface
that received the update?
Yes: Router 2 applies the mask /24, which is the mask of the interface that received the
update.
This process results in these networks and masks in the routing table of Router 2, displayed with the show ip
route command:
R 137.99.0.0/16 [120/1] via 131.108.2.2, 00:00:07, Serial
131.108.0.0/24 is subnetted, 3 subnets
R 131.108.5.0 [120/1] via 131.108.2.2, 00:00:08, Serial0
C 131.108.2.0 is directly connected, Serial0
C 131.108.3.0 is directly connected, Ethernet00
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Related Information
Why Don't RIPv1 and IGRP Support Variable-Length Subnet Mask?
Why Doesn't RIP or IGRP Support Discontiguous Networks?
