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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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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 ...

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