CISCO IOS in a Nutshell

Passive Interfaces

The passive-interface command tells an interface to listen to RIP or IGRP routes but not to advertise them. By disabling routing announcements on an interface, we tell the router to “listen but don’t talk.” This feature can reduce routing load on the CPU by reducing the number of interfaces on which a protocol needs to communicate. For OSPF and EIGRP, this command completely disables route processing for that interface. Use this command only if you know for sure that the routing protocol doesn’t need to talk to anything on the specified interface.

Figure 8-2. Using passive interfaces

In Figure 8-2, Router 1 and Router 2 can be optimized with the passive-interface command. Without getting into the routing protocol specifics, we want to say “Keep the EIGRP routing on the serial links and keep the RIP routing on the Ethernet interfaces.” Here’s how:

router eigrp 300
 network 192.168.10.0
 passive-interface ethernet0

router rip 
 network 192.168.10.0
 passive-interface serial0
 passive-interface serial1

This configuration tells Router 1 and Router 2 not to send any EIGRP updates over the Ethernet interface and not to send RIP updates over the serial links. It doesn’t solve the problem of getting routing information from our EIGRP process into RIP and vice-versa. We will solve this problem using route redistribution.

EIGRP normally multicasts route information to neighbor ...