Doubt on OSPF Virtual link

Unanswered Question
Mar 5th, 2006
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Hi all,

Kindly go through the attached picture.In that i have some questions to ask:

1.After VL is established between R2 and R3 through A1,whether router R1 uses R2 to reach Area 1 or R3 to reach Area 1.

2.What happens if all interfaces on R2 is disabled, whether R1 can reach Area 1

and Area 0?

3.What happens if all interfaces on R3 is disabled, whether R1 can reach Area 0 and Area 1 ?



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Overall Rating: 4 (3 ratings)
pkhatri Mon, 03/06/2006 - 00:01
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Hi Vijay,

In answer to your questions:

1. R1 will route all packets destined to area 1 to R2, since that is the ABR that advertises reachability to area 1 destinations to area 2. By creating the virtual link, R2 also becomes a backbone router. So there will not be any sub-optimal routing as such.

2. If all interfaces on R2 are disabled, there is really no way for R1 to get through R2, is there ? So area 2 will get completely partitioned from area 2 and area 0.

3. If all interfaces on R3 are disabled, the virtual link will go down. The behaviour here is really implementation-dependent. The Cisco ABR behaviour follows RFC3509 - 'Alternative Implementations of OSPF Area Border Routers'. As per this RFC, R2 in your example will not be considered an ABR and will not generate inter-area summaries into the 2 areas. Therefore, R1 will not be able to reach destinations in Area 1.

Hope that helps - pls rate the post if it does.


mheusinger Mon, 03/06/2006 - 00:02
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Hello Vjay,

A1) R1 will see R2 announcing itself as ABR (Area Border Router) and inserting area1 and area0 routes. Thus R1 will use R2 (the only way possible because of topology anyhow.)

A2) In case all interfaces in R2 are disabled (shutdown) there is no more connectivity between R1 and R3 (again, physical connectivity is not given any more)

A3) R1 can not reach area0 and area1, when R3 has all interfaces disabled because the VL will be down and also no more pysical access to area0 exists.

Hope this helps! Please rate all posts.

Regards, Martin

pkhatri Mon, 03/06/2006 - 00:19
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Another thing I should add: in an implementation compliant with RFC2328, the answer to your question 3 would be that R1 would be able to reach destinations in area 1, even if it had no connectivity to area 0.


vpalania Mon, 03/06/2006 - 02:10
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Thanks all for your explanation.

I was asking compliant with RFC2328.In that case, R1 still reach area1 through ABR R2.Am I correct ?



pkhatri Mon, 03/06/2006 - 02:14
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That is correct, Vijay. As per RFC2328, a router with attachments to more than one area is considered an ABR, regardless of whether or not it has an active adjacency to Area 0.

Hope that helps - pls rate the post if it does.



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