OSPF summarization

Route summarization helps reduce OSPF traffic and route computation. 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 understand OSPF summarization, consider the following example network:

ospf summarization topology

All three routers are running OSPF and exchanging routes. Before OSPF summarization is configured, 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.

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
     11.0.0.0/24 is subnetted, 2 subnets
O IA    11.0.0.0 [110/3] via 10.0.0.2, 00:00:40, FastEthernet0/0
O IA    11.0.1.0 [110/3] via 10.0.0.2, 00:00:40, FastEthernet0/0
O IA 172.16.0.0/16 [110/2] via 10.0.0.2, 00:00:40, FastEthernet0/0

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

Router(config)#router ospf 1
Router(config-router)#area 1 range 11.0.0.0 255.255.0.0

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

Router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
C       10.0.0.0 is directly connected, FastEthernet0/0
     11.0.0.0/16 is subnetted, 1 subnets
O IA    11.0.0.0 [110/3] via 10.0.0.2, 00:00:30, FastEthernet0/0
O IA 172.16.0.0/16 [110/2] via 10.0.0.2, 00:02:52, FastEthernet0/0
NOTE
Be careful with summarization. In this case, router R1 thinks that R2 has routes for all subnets in the range 11.0.0.0 – 11.0.255.255. When summarizing, try to be as specific as possible.

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