Type 3 LSA on ABR non-backbone area int

Unanswered Question
Feb 7th, 2007

We have stood up a new network with new area 0. We are trying to figure out the best way to transition the legacy area 0 and non-backbone areas onto the new core. One solution is to maintain separate area 0s. We are still discussing this strategy but one question that remains unanswered is if an OSPF ABR receives a type 3 LSA on a non-backbone area interface will it import it into the routing table?

I have this problem too.
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royalblues Wed, 02/07/2007 - 04:51

If i understand your topology

R1(area0)---R2(area2)---R3(area3)

What i have seen in the lab is, R2 receives the route for the networks connected to R3 and can be seen in the route table.

But the same are not advertised to the backbone and hence it is not reachable from there and networks connected to it.

This would require a Virtual-link or a GRE tunnel between R3 & R1

HTH, rate if it does

Narayan

dnewell24 Wed, 02/07/2007 - 05:01

Narayan,

RTR1(area1)---(area1)RTR2(area0)-Legacy-(area 0)RTR4(Area4)

RTR1(area1)---(area1)RTR3(area0)---New--RTR5

RTR1 would have a connection to RTR2 and RTR3 over area 1. RTR 2 and RTR 3 would also have a connection over area 1. Area 0s are disconnected.

Assuming the LSA is type 3, would routes learned from RTR 4 enter RTR 3 routing table. In most cases type 3 routes are usually learned through an area 0 interface. In this case it comes in on area 1, a non-backbone interface.

Thanks for you help.

mheusinger Wed, 02/07/2007 - 05:49

Hi,

an ABR will ignore an LSA3 learned through a non- backbone area interface. This is one of the underlying reasons, that area0 MUST be contigous. Assume R1-area0-R2-area1-R3-area0-R4

In this case networks from R1 are learned by R3 through LSA3 from ABR R2. R3 has the LSA3 in its topology database from area1. But R4 will never get the networks as R3 will not insert the LSA3 information into area0.

From another point of view: if you have redundant ABRs, routing loops could arise, if LSA3 from a non-backbone-interface would be reinserted into the backbone area.

Regards, Martin

royalblues Wed, 02/07/2007 - 05:54

As i posted earlier and confirmed by martin, the routes learned by router R3 will not be learned by R4

HTH

Narayan

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