ospf, areas, and ABR's

Unanswered Question
Aug 18th, 2008

Can anyone advise please, on guidelines for number of abr's from an area to area 0. In most literature you see 1 abr between an area and backbone, but can you have more than this and what if any are the consequences?



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Edison Ortiz Mon, 08/18/2008 - 06:20

You can safely have more than one ABR in an area. I recommend having at least 2 and no more than 3.




reidg Mon, 08/18/2008 - 06:33

Edison, thanks for the reply.

can you expand on why more than 3 is a problem?


Edison Ortiz Mon, 08/18/2008 - 06:54

You are introducing more 'gateways' in/out of your non-zero areas and traffic flow can be unpredictable which can cause some asymmetrical packet forwarding.

For instance, a router in a non-zero area may choose ABR 1 to exit the area but the return traffic may come via ABR 2 or 3.

With 2 ABRs, is somewhat acceptable since it provides some fault tolerance for those non-zero areas.




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reidg Mon, 08/18/2008 - 07:15

Okay, A scenarion then:

2 abr's between an area and the backbone, both abr's with same connectivity to said backbone.

All routers that existing within the area exist on the same subnet as do the abr's, lets say it a 10mb metro ethernet.

So they will all see two routes to the backbone, with similar values, how will they choose which one to send traffic via.



reidg Mon, 08/18/2008 - 11:30

yeh but there must be a preference mechanism?

Joseph W. Doherty Mon, 08/18/2008 - 17:48

One situation you can run up against with multiple ABRs, if the ABRs are advertising a area summary address, an ABR can receive traffic from area 0 that it doesn't have a path to, or a very suboptimal path. (Possible solution: have a "local" path between ABRs that's within the ABR's non-backbone area.)


Regarding your question about path preference to ABRs, much the same as OSPF path selection in general, i.e. often summary of links costs from any point to the ABR.


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