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

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Summarizing Routes in OSPF

Problem

You want to reduce the size of your routing tables without losing any connectivity within your network.

Solution

Using the area x range configuration command on your ABRs allows you summarize routes between OSPF areas:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router ospf 55
Router1(config-router)#area 100 range 172.20.0.0 255.255.0.0
Router1(config-router)#area 0 range 172.25.0.0 255.255.0.0
Router1(config-router)#area 2 range 10.0.0.0 255.0.0.0
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

The easiest way to see the effect of summarization is to look at the routing table before and after it is enabled. Here is a sample routing table before summarization. The ranges that we will be summarizing are 172.20.0.0/16 and 172.25.0.0/16. We have highlighted the route with the lowest metric in each of these ranges:

Router2#show ip route ospf 172.16.0.0/24 is subnetted, 1 subnets O IA 172.16.2.0 [110/140] via 10.1.1.2, 00:05:06, Serial0/0 172.20.0.0/16 is variably subnetted, 3 subnets, 3 masks O IA 172.20.10.0/24 [110/1702] via 10.1.1.2, 00:05:06, Serial0/0 O IA 172.20.1.0/30 [110/1692] via 10.1.1.2, 00:05:06, Serial0/0 O IA 172.20.100.1/32 [110/1693] via 10.1.1.2, 00:05:06, Serial0/0 172.25.0.0/16 is variably subnetted, 3 subnets, 2 masks O IA 172.25.25.6/32 [110/141] via 10.1.1.2, 00:05:06, Serial0/0 O IA 172.25.25.1/32 [110/131] via 10.1.1.2, 00:05:06, Serial0/0 O IA 172.25.1.0/24 ...

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