Cisco IOS Cookbook, 2nd Edition

EIGRP Route Summarization

Problem

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

Solution

The ip summary-address eigrp configuration command allows you to configure manual summary addresses on a per-interface basis:

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 eigrp 55 172.25.0.0 255.255.0.0
Router1(config-subif)#exit
Router1(config)#end
Router1#

EIGRP can automatically summarize subnet routes into classful network-level routes. You can enable this command with the auto-summary command or disable it with the no auto-summary configuration command:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#no auto-summary
Router1(config-router)#exit
Router1(config)#end
Router1#

A useful new feature allows you to configure a leak-map so that the router will advertise the summary route, as well as some subset of the summarized addresses:

Router9# configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router9(config)#ip prefix-list 10.5.5/24 permit 10.5.5.0/24
Router9(config)#route-map LEAK10-5-5 permit 10
Router9(config-route-map)#match ip address prefix-list 10.5.5/24
Router9(config-route-map)#exit
Router9(config)#interface Serial0/0
Router9(config-if)#ip summary-address eigrp 55 10.5.0.0 255.255.0.0 ...