OSPF route summarization

Unlike EIGRP, OSPF doesn’t support automatic summarization. Also, unlike EIGRP, where you can summarize routes on every router in an EIGRP network, OSFP can summarize routes only on ABRs and ASBRs. Route summarization helps minimizes OSPF traffic and reduces route computation.

The following command is used for summarization in OSPF:

(config-router) area AREA_ID range IP_ADDRESS SUBNET_MASK

To better understand OSPF summarization, consider the following example:

ospf summarization example

In the example above we have a network of three routers and two hosts. All three routers are running OSPF and are exchanging information. Before OSPF summarization is configured, the router R3 inside the area 1 has two entries for the subnets 10.0.0.0/24 and 10.0.1.0/24 in its routing table:

show ip route ospf without summarization

We can summarize these two subnets on R2 so that R3 would have only one route for both subnets. To do that, the following command can be used on R2:

ospf summarization area command

Now, R3 will have only a single entry in its routing table for R1’s directly connected subnets:

show ip route ospf summarization

 

Be careful when summarizing routes. In this case, the router R3 thinks that R2 has routes for all subnets in the range 10.0.0.0 – 10.0.255.255. When summarizing, try to be as specific as possible.

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