OSPF ABR question

Unanswered Question
Dec 2nd, 2007

I have router A and Router B connected together.

I have Router C connected to both Router A and Router B in a triangle design.

Router A has a link to Router C. Area 10

Router B has a link to Router C. Area 10

What will be the ABR for Area 10? Where do I summarize the routes for the locally connected networks on Router C?

Please advise.

I have this problem too.
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Edison Ortiz Sun, 12/02/2007 - 15:50

You haven't mentioned any OSPF area other than Area 10. You need 2 OSPF areas in a router to be considered an ABR.

Richard Burts Sun, 12/02/2007 - 19:11


The point made by Edison is very true, that you have mentioned only area 10, and with only a single area defined there is no ABR.

To supplement his response a bit, in the Cisco implementation, it is not enough that a router have interfaces in 2 areas, to be an ABR it is also necessary that the ABR have an interface in area 0. The interface in area 0 can be a physical interface or it can be a virtual link. But a router with intefaces in 2 areas where neither area is area 0 (for example a router with interfaces in area 10 and in area 20) does not function as an ABR.



bvsnarayana03 Sun, 12/02/2007 - 19:54

There is no mention of area between A & B. If they are also in area 10 & you need to create an ABR among the 3, then you may create a loopback on router C & advertise it in area 0. This would make C an ABR & you may summarise local networks of C.

cisconoobie Mon, 12/03/2007 - 13:30

Router A and Router B have a 3 SVI's between them and are in Area 0.

I plan on connecting Router D to Router A and B also to form Area 20.

Is there any reason I should put Router A & B also in area 10? I need a backbone area 0, I thought.

Edison Ortiz Mon, 12/03/2007 - 17:51

> Is there any reason I should put Router A & B also in area 10?

If they plan to be OSPF neighbor with Router C, they need to have Area 10 as well.

Are you coming up with things as the day goes along ? :)

paul.matthews Tue, 12/04/2007 - 06:47

If I understand this correctly, you have a pair of routers - A&B with three logical links between them.

You then have two more routers. C&D. These both link back (dual homed to A&B



C ||| D



The three links between A&B are area 0 - that is yor backbone. The links to/from C are in area 10 and D area 20.

That makes A&B your ABRs.

A network like this would I hope just be a lab excercise, as there is little point running areas with just 4 routers. The summaristation would be configured on A&B, but all it would acheive would be smaller routing tables on C&D - whatever the summaries are for the "far" area, ans whatever you summarised the three backbone links as.

cisconoobie Tue, 12/04/2007 - 15:00

Ok good to know because I have about 20 different networks on router c and will have about 20 on router D.


So I dont want to have 20 network entries via network C, instead I want it summarized.



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