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 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
To summarize networks learned by redistribution, use the OSPF command summary-address. You can probably do this summarization in your head, but do so before continuing with the lab.
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. Note that the summary route is still marked as an E2 route.
There's an interesting route installed into R1's OSPF table as well.
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....
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, 00:03:10, Null0
O 12.0.0.0/6 is a summary, 00:07:53, Null0
.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.
To be successful on the BSCI exam and in earning your CCNP, you've got to master route redistribution. This isn't as easy as it sounds, because configuring route redistribution is only half the battle. Whether it's on an exam or in a real-world production network, you've got to identify possible points of trouble before you configure route redistribution - and you need to be able to control redistribution as well. You may have an OSPF domain with 100 routes, but only need to redistribute 10 of them into a neighboring RIPv2 domain. You've got to know how to do that, and one method is the use of a distribute-list.
A distribute-list is an access-list that is used to determine what routes can and cannot be redistributed. Distribute-lists let you specify what routes will be filtered from the process. You can use standard or extended ACLs, and you can filter routes that are coming into a routing process or being injected into another process.
In the following example, R1 is redistributing RIP routes into OSPF, but only wants to advertise network 150.1.1.0 /24 to other OSPF routers. An ACL will be written to match that particular network, and then the distribute-list will be written under the routing process. I'm going to show you the IOS Help output for the distribute-list command, and please note that routing updates can be controlled at the interface level or protocol level.
Using distribute-lists does guard against routing loops, but they have other purposes. You may have a network segment that should be kept secret from the rest of your company; a distribute-list can filter that segment's network number from the redistribution process. In this way, distribute-lists serve as a basic form of network security. (Very basic. I wouldn't sell that firewall on ebay if I were you.)
Keeping such networks out of routing updates and routing tables throughout the network has the side effect of reducing routing update overhead as well.
Chris Bryant has sinced written about articles on various topics from CISCO CCNA, Personal Desktop and Cisco CCNP. Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free and. Chris Bryant's top article generates over 27100 views. to your Favourites.