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The success of your BSCI and CCNP exams depends on the understanding of the details, one of which is the right way to understand the routing in OSPF. The route summary is not just a test of the binary conversion capability, but also where and when to aggregate the route. No CCNA or CCNP certified candidates are surprised, and OSPF provides us with the most routing aggregation options, so you need to learn more!
OSPF provides two options for routing summary configuration. In the previous tutorial, we looked at the "summary-address" command, and today we will look at the correct use of the "area range" command.
"area range" commands should be used in the Area Border Router (ABR) to summarize the advertised routes from one OSPF area to another. In this tutorial, R1 acts as an ABR and has an interface in both area 0 and area 1. Four loopback is placed in area 1 in R1.
R1(config)#router ospf 1
R1(config-router)#network 12.0.0.0 0.255.255.255 a 1
R1(config-router)#network 13.0.0.0 0.255.255.255 a 1
R1(config-router)#network 14.0.0.0 0.255.255.255 a 1
R1(config-router)#network 15.0.0.0 0.255.255.255 a 1
The routing table of an OSPF neighbor, R2, shows all four routes.
R2#show IP route OSPF
12.0.0.0/32 is subnetted, 1 subnets
O IA 12.12.12.12 [110/65] via 172.12.123.1, 00:18:52, Serial0
13.0.0.0/32 is subnetted, 1 subnets
O IA 13.13.13.13 [110/65] via 172.12.123.1, 00:18:42, Serial0
14.0.0.0/32 is subnetted, 1 subnets
O IA 14.14.14.14 [110/65] via 172.12.123.1, 00:18:32, Serial0
15.0.0.0/32 is subnetted, 1 subnets
O IA 15.15.15.15 [110/65] via 172.12.123.1, 00:18:32, Serial0
To keep the routing table of the downstream router small but still have the required IP connection, we can summarize the four routes using the area range command on R1. The key thing to remember with the range command is that the region number given in the command is the region that contains the destination, not the region that will receive the summary route.
R1(config)#router ospf 1
R1(config-router)#area 1 range 12.0.0.0 252.0.0.0
R2 now shows a single summary route that can be used to reach all four remote networks.
R2#show IP route OSPF
O IA 12.0.0.0/6 [110/65] via 172.12.123.1, 00:00:21, Serial0
Interestingly enough, there's now an additional route in R1's routing table.
R1#show IP route OSPF
O 12.0.0.0/6 is a summary, 00:07:53, Null0
When the summary route is configured in OSPF, the route to null0 is installed in the OSPF routing table of the router performing the summary. This helps prevent routing loops. Any packets sent to the summarized route will have longer matches in the routing table, and packets that match one of the summary routes but match the summary route will be dropped.
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