ABR between non backbone areas??

Answered Question
Nov 24th, 2007

Hi all,

I have a question about designing an OSPF network: suppose that I have two non backbone areas , for example areas 1 and 2, conected to backbone area 0.(normal behaviour).

And suppose that the network is geographically growing in a way that makes the two areas 1 and 2 adjacent. What would be the behaviour if I conect the two areas directly? (I would have an ABR between areas 1 and 2, and these areas are of course still conected to area 0).I am not sure about the fact that LSAs will still be transmitted through this ABR as summary LSAs...

Best regards,

brahim

I have this problem too.
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Correct Answer by s.arunkumar about 9 years 1 month ago

In ospf its a must to have all area to be connected to backnone area to avoid the looping by using the distance vector behaviour exhibited by updates from the abr of backbone and non-backbone area.As above mentioned virtual link can be used to manipulate this..

I came across something called "Alternate ABR" which also modifies the abr behaviour and thus enabling two non-backbone area to be connected via router acting as abr..

think this is relevent to dicussion and would help

http://www.faqs.org/rfcs/rfc3509.html

Correct Answer by Richard Burts about 9 years 1 month ago

brahim

Yes if you have a router with interace(s) in area 1 and interface(s) in area 2 then you need a virtual link to make this router function as an ABR. While you could do 2 virtual links, you really only need 1. As long as the router is connected to area 0 once that is enough.

HTH

Rick

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Richard Burts Sat, 11/24/2007 - 10:19

brahim

I am not sure that I fully understand your question. I believe that you are saying that you have an area 0, you have an area 1 with an ABR which connects area 1 to area 0, and that you have an area 2 with an ABR which connects area 2 to area 0. I believe that the question is if you install a router that has a connection in area 1 and has a connection in area 2, would that router function as an ABR and propagate LSAs from area 1 into area 2? That router would not function as an ABR in the Cisco implementation. In Cisco's implementation to function as an ABR a router must have an interface to area 0 before it will act as an ABR and generate summary LSAs between areas. Note that if the router is configured with a virtual link which establishes a virtual interface to area 0 then the router would function as an ABR. But a router with a physical connection to area 1, and a physical connection to area 2, but no connection to area 0 will not function as an ABR.

HTH

Rick

sagou.brahim Sat, 11/24/2007 - 10:37

Hi Rick,

Thanks for the answer it corresponds exactly to my question. So in this particular case, if I want this router to act as ABR for both areas 1 and 2 I would have to set 2 virtual links from this router: one to conect area 1 to area 0 (virtual link through area 2) and another one to conect area 2 to area 0 (virtual link through area 1)... well it sounds a little weird but It is about a customer network which can arrive to this situation: two areas which will be directly conected, so we'd like to set up the redundant paths (traffic from area 1 flowing through area 2 if its ABR crashs or something...)

many thanks

brahim

Correct Answer
Richard Burts Sat, 11/24/2007 - 13:53

brahim

Yes if you have a router with interace(s) in area 1 and interface(s) in area 2 then you need a virtual link to make this router function as an ABR. While you could do 2 virtual links, you really only need 1. As long as the router is connected to area 0 once that is enough.

HTH

Rick

Correct Answer
s.arunkumar Sun, 11/25/2007 - 22:36

In ospf its a must to have all area to be connected to backnone area to avoid the looping by using the distance vector behaviour exhibited by updates from the abr of backbone and non-backbone area.As above mentioned virtual link can be used to manipulate this..

I came across something called "Alternate ABR" which also modifies the abr behaviour and thus enabling two non-backbone area to be connected via router acting as abr..

think this is relevent to dicussion and would help

http://www.faqs.org/rfcs/rfc3509.html

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