Cisco IOS Cookbook, 2nd Edition

Redirecting ICMP with HSRP

Problem

You want to enable ICMP redirects with HSRP.

Solution

In older IOS releases, when you enable HSRP on an interface, the router will automatically disable ICMP redirection. However, starting with IOS Version 12.1(3)T, Cisco has changed how ICMP redirection works with HSRP, and it is now enabled by default.

You can explicitly enable ICMP redirects on HSRP-enabled interfaces with the following commands:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface FastEthernet 1/0
Router2(config-if)#standby redirects enable  
Router2(config-if)#exit
Router2(config)#end
Router2#

The following commands prevent the router from the sending ICMP redirects on HSRP-enabled interfaces:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface FastEthernet 1/0
Router2(config-if)#no ip redirects           
Router2(config-if)#standby redirects disable 
Router2(config-if)#exit
Router2(config)#end
Router2#

The unknown keyword allows you to use ICMP redirection to nonHSRP routers:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface FastEthernet 1/0
Router2(config-if)#standby redirects unknown 
Router2(config-if)#exit
Router2(config)#end
Router2#

Discussion

When a router receives a packet from a LAN interface, but the route to the destination points to another router on the same LAN segment, the router will send an ICMP Redirect ...