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ordered SPFs

Subject: ordered SPFs
From: Alia Atlas
Date: Wed, 20 Oct 2004 01:12:53 -0400
Hi Mike & Stewart,

I was taking another look at the Ordered SPF idea and what you've written in draft-bryant-shand-lf-conv-frmwk-00.txt and have a few comments/questions.
First, I'd picked up the idea (looking through emails I'm not sure where :)
that you'd calculate a reverse spanning tree from each router connected to
the failure and wait based on the minimum hops. It sounds like you are
actually calculating from the failed component itself. For a
point-to-point link failure, I don't think it matters, but for a node or
broadcast link failure, it definitely does. I think that you may need the
same thing when computing for a new link coming up. More details,
particularly on the ECMP cases would be useful.
There's also a bit more complexity to the reverse spanning tree in terms of
what paths are inherited in the ECMP case. If you just break ties based on
IP address or keep the first shortest path that's found, you could end up
with rather bad results. Also, you'd need to keep track of the highest
hop-count on a shortest path to know how long to wait.
I do see why the delay isn't computed based on destination; all the
affected traffic took the same shortest paths to the failed component (or
will take the same shortest paths to the added component).
Where I think this method may have a problem is with SRLGs. For what I've
been calling local SRLGs (all members connected to the same router), there
wouldn't be an issue, b/c you could pretend that the node had failed from
the perspective of rooting your reverse spanning tree. For more general
SRLGs, there would be a problem. At a minimum, one would need to do
destination-based delay and determine which destinations were reached via
which failed element(s). One would also have to decide how to handle
destinations which were reached via equal-cost paths, where each equal-cost
path went via a different failed element. One would need to couple the
separate reverse spanning trees at the point where the ECMP existed and
bump up the hop-count equivalently (or something similar). Have you
thought about the implications of SRLGs?
I realize that we have not been discussing SRLGs substantially in the
context of IP FRR. I think this is just b/c of wanting to get the basics
down first, before starting to embellish with all the details. It is, of
course, completely possible to consider SRLGs. This can just be an
additional factor in the acceptance criteria of a loop-free alternate; the
same thing works for U-turn alternates. The concern would be the
additional computation such tracking would require.
Another issue, of course, is how much engineering current networks would
require to have protection if important SRLGs were considered.
What do you think?


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