Cisco IOS Cookbook, 2nd Edition

RIP Route Summarization

Problem

You want to decrease the size of your routing tables to improve the stability and efficiency of the routing process.

Solution

You can manually configure address summarization on an individual interface with the ip summary-address rip configuration command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0/0.2 
Router1(config-subif)#ip summary-address rip 172.25.0.0 255.255.0.0
Router1(config-subif)#exit
Router1(config)#end
Router1#

By default, RIP will summarize groups of subnets into classful network routes. You can disable this automatic summarization with the no auto-summary configuration command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router rip
Router1(config-router)#no auto-summary
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

RIP automatically summarizes along classful network boundaries. So if your router sees that several subnets of the same network all use the same path, and there are no subnets of this network using a different path, it will automatically summarize this information. The routes to the individual subnets are suppressed. Then any downstream devices from this router will see only a summary route, such as 172.25.0.0/16 instead of all of the individual subnets like 172.25.1.0/24, 172.25.2.16/28, and so forth.

The downstream devices don’t need to know that they are seeing summary ...