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

Subject: Re: LFAs and OSPF with multiple intra-area AS-external or summary-routes
From: Alex Zinin
Date: Wed, 9 Feb 2005 17:25:39 -0800
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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