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Re: LFAs and OSPF with multiple intra-area AS-external or summary-route

Subject: Re: LFAs and OSPF with multiple intra-area AS-external or summary-routes
From: Alia Atlas
Date: Wed, 09 Feb 2005 20:53:44 -0500
Alex,

One thing to note is that the problem can't occur if D_opt(N, D) < D_opt(E, D). I.e., if the alternate next-hop is a downstream path with regard to the primary next-hop. Not that this helps with coverage :-(
Similarly, if no other ABR in the area is announcing a shorter path than
D_opt(E,D), then it couldn't happen. Or even if the combo of the distance
from the other ABR to E plus the path announced is shorter than
D_opt(E,D). If S is an ABR, then S could hear all the other ABR's summary
routes and be able to determine this, I think. Not that it's pretty...
Alia

At 08:25 PM 2/9/2005, Alex Zinin wrote:
Alia,

>>Thanks.
>>In this scenario, assuming the failure is that of P1 node, Y would lose its
>>connectivity in area 1 completely, wouldn't it?

> True.  One could easily add a link between Y and another node in area 1 so
> that doesn't happen. There could be a link between S and Y of greater cost
> - so that Y tries to use S as a loop-free link-protecting alternate.

Understood.

I thought about this today. It seems that in the above situation, for a loop to form, S doesn't even have to be a direct neighbor of Y, just belong to the path
rooted at Y's LFA. In fact, it also appears that S doesn't even have to do
ECMP...

I need to think more about this, but it seems that the LFA condition for ABRs
cannot be simply based on a single area topology. I'll provide more details when
I'm done going through different scenarios.

Of course, the last resort is to say that transit areas are not supported yet,
but I want to make sure we understand how hard it would be to get them to work
first.

Alex


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