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Re: ordered SPFs

Subject: Re: ordered SPFs
From: Alia Atlas
Date: Wed, 20 Oct 2004 12:25:31 -0400
Mike,

At 08:05 AM 10/20/2004, mike shand wrote:
Yes. In each case you compute the reverse SPF rooted at the far end of the failed link as indicated in the LSP you are considering. A node failure does not appear as a single event, but causes multiple LSPs to be sent (1 for each link failure). Each of these is considered as for a link failure. It can be shown that in the special case of an "SRLG" which is a node failure, there can be no conflict in the required ordering of SPFs.
How do you combine/determine the node position to use if there are multiple
trees? I assume that it must be the furthest away hopCount (for the
failure case) and, if a node were to instantly come up with multiple links,
then it would be the closest hopCount (for the recovery case).
It would also be very good to specify what topology the RSPT is being
calculated on. I.e., this requires that one store the old topology, which
is a slightly different complication.
Note that there is a general problem with (I think) all the methods we are talking about for loop prevention and repair, where you have routers which are imposing different ECMP limits. Any calculation should be made assuming the "worst" case which is the largest number of path splits.
I don't think that one needs to assume the "worst" case; I think you just
need correct inheritance rules within the SPF. Certainly, that is what I
have for the loop-free and for the U-turn alternates.
The problem otherwise isn't just with different ECMP limits, but with
different tie-breaking criteria within those.
Yes. Clearly it is possible for unrelated SRLG (as opposed to the special case of node failure), to introduce conflicting ordering requirements on the whole FIB. As you say this would introduce a lot of extra complexity, and require per destination (or at least per destination set) ordering of the FIB.
Do you see this as a problem or as a reasonable trade-off given we have
per-destination hold-down for the local repair?
Also, how would you combine hopCounts from different trees? Can this work
for 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.
Absolutely. The other problem with SRLG as far as repair is concerned is
that you cannot be sure about your SRLGs. On the one hand you may have a
failure of a single link which is part of an SRLG, and hence you may
invoke a more complex repair strategy that actually required (or worse,
decide that the failure was irreparable, when in fact the single link
failure could have been repaired). On the other hand, you may get multiple
link failures occurring as a result of a real world SRLG of which you were
previously aware.
Sure, it's a question of how paranoid does one really want to be. Do you
always use an alternate (if available) that will provide SRLG protection?
I tend to be in the paranoid in planning (i.e. get the most protective
alternate you can) and simplistic in the set of repair strategies. A
separate one for link & node is not unreasonable, given BFD to make a very
quick determination. Beyond that, I don't see how a router could know
about an SRLG-caused failure in the appropriate time frame.
Alia


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