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OSPF Summarization

Route summarization helps decrease OSPF traffic and route forming. OSPF, unlike EIGRP, 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.

The following command is used for OSPF summarization:

(config-router) area AREA_ID range IP_ADDRESS MASK

To better explain OSPF summarization, consider the following example network:

All three routers are running OSPF and swap routers. Before OSPF summarization is setup, the router R1 inside the backbone area has two entries for the networks 11.0.0.0/24 and 11.0.1.0/24 in its routing table.

We could summarize these two subnets on R2, so that R1 receive only one routing update for both subnets. To do that, the following command can be used on R2:

Now, R1 has only one entry in its routing table for R3’s directly connected subnets:

 

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