Cisco IOS Cookbook, 2nd Edition

Applying Offsets to Routes

Problem

You want to modify the routing metrics for routes learned from or sent out through a particular interface.

Solution

You can modify the RIP metrics for a list of routes learned through a particular interface with the offset-list configuration command:

Router2#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#access-list 22 permit 192.168.20.0 
Router2(config)#router rip
Router2(config-router)#offset-list 22 in 5 Serial0.1
Router2(config-router)#exit
Router2(config)#end
Router2#

A similar command changes the metrics for a list of routes as they are sent out through a specified interface:

Router2#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#access-list 33 permit 192.168.30.0
Router2(config)#router rip
Router2(config-router)#offset-list 33 out 10 Serial0.1
Router2(config-router)#exit
Router2(config)#end
Router2#

Discussion

The offset-list command is most useful when you need RIP to take account of the costs of different links. By default, RIP only looks at the number of hops to the destination. But sometimes the longer path is significantly faster. For example, you might have a primary link that uses a T1 to get to a router in another building, and an Ethernet segment to get from there to another router that connects to a server. It’s probably better to take this primary link than a backup 56Kbps circuit that happens to connect directly to the last hop router. ...