Cisco IOS Cookbook, 2nd Edition

Unicast Updates for RIP

Problem

You want to exchange routing information with one device on a network, but not with any others.

Solution

You can configure RIP to send its updates to a neighboring router using unicast instead of broadcast or multicast packets. This is useful in two situations. First, on Nonbroadcast Multiple Access (NBMA) networks, you can’t use the standard broadcast or multicast methods for distributing information because the media doesn’t support it. Second, sometimes you need to exchange routing information with one or more specific devices on a segment, but you don’t trust the rest to give you reliable information. This feature is rarely used, but it can be extremely valuable in these types of situations:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router rip
Router1(config-router)#passive-interface FastEthernet0/1
Router1(config-router)#neighbor 172.22.1.4
Router1(config-router)#end
Router1#

Discussion

This recipe uses the passive-interface command discussed in Recipes 6.6 and 6.7 to prevent the router from sending routing information to the interface in general. Note that it does not prevent the router from receiving routing information from other devices on the segment. We will discuss how to solve that problem in a moment.

A debug trace helps to show how the unicast update option works:

Router1#debug ip rip
RIP protocol debugging is on
Router1#
Aug 11 02:41:13.632: RIP: sending v1 update to 255.255.255.255 ...