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.
There are no specific requirements for this document.
The information in this document applies to these software and hardware versions:
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.
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
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.
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:
When Router 1 sends an update to Router 2, it performs these checks:
Is 188.8.131.52/24 part of the same major net as 184.108.40.206/24, which sources the update?
Is 220.127.116.11/24 part of the same major net as 18.104.22.168/24, which sources the update?
This process results in Router 1 including 22.214.171.124 and 126.96.36.199 in its update to Router 2. You can see this in the debug ip rip command output shown on Router 1:
*Mar 25 00:22:46.177: RIP: sending v1 update to 255.255.255.255 via Serial0 (188.8.131.52)
*Mar 25 00:22:46.178: RIP: build update entries
*Mar 25 00:22:46.182: subnet 184.108.40.206, metric 1
*Mar 25 00:22:46.185: network 220.127.116.11, metric 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 18.104.22.168 on Serial0
*Mar 25 00:22:46.203:22.214.171.124 in 1 hops
*Mar 25 00:22:46.205:126.96.36.199 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 188.8.131.52 the same as 184.108.40.206, which is the address assigned to the interface that received the update?
Does subnet 220.127.116.11 belong to the same major net as subnet 18.104.22.168, which is 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 22.214.171.124/16 [120/1] via 126.96.36.199, 00:00:07, Serial0
188.8.131.52/24 is subnetted, 3 subnets
R 184.108.40.206 [120/1] via 220.127.116.11, 00:00:08, Serial0
C 18.104.22.168 is directly connected, Serial0
C 22.214.171.124 is directly connected, Ethernet0