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Re: ordered SPFs

Subject: Re: ordered SPFs
From: Alia Atlas
Date: Fri, 22 Oct 2004 08:51:28 -0400
At 07:49 AM 10/22/2004, mike shand wrote:
At 01:18 22/10/2004 -0400, Alia Atlas wrote:
To clarify a bit further on the inter-area issue, does this handle the case where the cost of a route advertised into the area changes so that a different ABR is selected?
I hadn't thought about that, but off hand I would say that you can model
this scenario as if you have two (or more) ABRs (say A and B) each of
which has a "link" to the external route X. So the scenario you describe
is equivalent to a failure (or cost change) of the AX or BX link. As such,
the normal mechanisms should handle it. But it would require special
casing to for the code to make this interpretation.
Such would be necessary to prevent micro-loops for inter-area routes; those
matter too :-)
I don't think that the distance to the prefix matters, so the same trick
could work for inter-AS routes, where what changes is whether there is
 What if the cost to the different ABR changes semi-simultaneously?
I think that is a case of the generalized SRLG isn't it?
Well, the key is that the costs could only increase or decrease together,
due to a single link or node failure, so if one does the fake node trick,
then you get the correct RSPT for ordering.
First, how does this interact/work with OSPF Virtual Links?
Virtual Links seem to present much more of a problem. Only the routers
connected to the virtual link knows the current length of it in hops. All
the other routers would probably assume it was a single hop.
Any thoughts on that?


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