OSPF "summarization" of "branch" routers

[markmonteleone_2]

Have a setup where 10 remotes sites are operating in area 1. They each advertize 4 subnets, which I have designed in 22bit allocations.

I know that summary statement usually in used in reference to the branch router operating as an ASBR from another routing protocol;

But is there an easy way to summerize the 4 subs that are directly attached to the router?

I tried the summary statement under ospf process, but it didn't seem to affect the routing table of any of the routers, in area 1 or Area 0.

Any suggestions allowing my branch sites to advertize a /22 so that I dont have an addtional 40 subs in table?

[answanso]

Hi Mark,

Unlike EIGRP, you cannot summarize in any location within the network with OSPF. There are two places where summarization can occur, the first is on an ABR and the second is on an ASBR. So in order to summarize into area 0, you would have to do on the router where area 0 and 1 touch. If the remotes are in a hub and spoke topology, you could extend area 0 to the hub router, and configure the links between the hub and spoke in area 1. You would then be able to summarize on the hub device as it would be an ABR. You would use the area area-id range ip-address mask command to accomplish this.

For more information please check out this link:

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t29

HTH

Anthony

[Richard Burts · Accepted solution]

Mark

Anthony is quite correct that OSPF is more restrictive about where summarization can be done as compared to EIGRP. I believe that there are at least two scenarios where you can accomplish what you want while using OSPF.

1)

- on the remote router run OSPF and have a network statement that matches only the link from the remote router to the central router.

- in OSPF redistribute connected subnets. This makes the remote router into an ASBR and will allow you to summarize the remote subnets on the remote router.

2)

- on the remote router run OSPF and have a network statement that  matches only the link from the remote router to the central router.

- on the remote router configure a static route (perhaps a route to null 0 or pick some next hop that works) that covers the range of the subnets on the remote router.

- in OSPF redistribute static. This will advertise the static which automatically provides summarization of the remote router subnets.

HTH

Rick

[markmonteleone_2]

Rich, I think you're on to something. I had done this a couple years but on some Foundry/brocade gear. I think that's how I accomplished it. I will try tomorrow and let you know.

Thanks either way.

[markmonteleone_2]

Thanks Rick, that did the trick. I dont like that as I may not have the granual ability to restrict a connected sub, but that did the trick.

I have added the " ip ospf cost 2000" statement at all OSPF interfaces and in the router ospf # region of the config and still cant seem to affect the cost. Prior to the connected statement, it seems the router daemon was using the interface statement to add the cost. How is that done now?

[Joseph W. Doherty]

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When you inject the networks as externals, interface cost usage depends on what type of external you're injecting the networks as.  The default type is "2", which does not consider interface cost.  Try adding "metric-type 1" to your OSPF redistribution statement.

[markmonteleone_2]

Joseph, thanks for your reply

Disclaimer: If you read this you must simile : )

I am sorry I didn't mention that I have the type setup as 1 already

router ospf 1

area 1 default-cost 2000

summary-address 10.128.4.0 255.255.252.0

redistribute connected metric-type 1 subnets

network 10.77.1.21 0.0.0.0 area 1