BSCI exam success, not to mention earning your CCNP, can come down to your OSPF route summarization skills. There are a few different commands and situations you need to be ready for, and one of these vital details is the proper use of the "summary-address" command.

The summary-address command should be used on an ASBR in order to summarize routes that are being injected into the OSPF domain via redistribution. In the following example, four routes are being redistributed into OSPF on R1, making R1 an ASBR.

interface Loopback16
ip address 16.16.16.16 255.0.0.0
!
interface Loopback17
ip address 17.17.17.17 255.0.0.0
!
interface Loopback18
ip address 18.18.18.18 255.0.0.0
!
interface Loopback19
ip address 19.19.19.19 255.0.0.0

R1(config)#router ospf 1
R1(config-router)#redistribute connected subnets

These four routes are seen on the downstream router R2 as External Type-2, the default for routes redistributed into OSPF.

R2#show ip route ospf
O E2 17.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 16.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 19.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0
O E2 18.0.0.0/8 [110/20] via 172.12.123.1, 00:00:07, Serial0

You can probably do this summarization in your head, but do so before continuing with the lab. : )

R1(config)#router ospf 1 R1(config-router)#summary-address 16.0.0.0 252.0.0.0

Look at the difference in R2's OSPF table.

R2#show ip route ospf
O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0

The external routes have been successfully summarized, and four routes have been summarized into one single route. Note that the summary route is still marked as an E2 route.

There's an interesting side effect from the summarization present on R1:

R1#show ip route ospf
O 16.0.0.0/6 is a summary, 00:01:51, Null0

When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table. This helps to prevent routing loops. Any packets destined for the routes that have been summarized will have a longer match in the routing table....

R1#show ip route

Gateway of last resort is not set

C 17.0.0.0/8 is directly connected, Loopback17
C 16.0.0.0/8 is directly connected, Loopback16
C 19.0.0.0/8 is directly connected, Loopback19
C 18.0.0.0/8 is directly connected, Loopback18
O 16.0.0.0/6 is a summary, 00:01:51, Null0

.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.

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