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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 20:45:41 -0500
Alex,

At 08:25 PM 2/9/2005, Alex Zinin wrote:
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...
Yes... Basically, the loop-free check doesn't have enough information.

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.
Looking forward to it :-)  I'm trying to do the same.

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.
I think there is still a problem with the AS-external as well. For
instance, say S has two equal-cost paths in area 1 to E1 and E2. On the
path from E2 to the prefix advertised by ASBR1, let it cross an ABR whose
shorter path is through area 2 to ASBR2. Then, let that path cross another
ABR whose shorter path is through area 1 via E1 (or the link to E1 even).
Maybe there's a restriction about LFAs passing through ABRs :-(

Alia


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