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Re: node disjoint and SRLG

Subject: Re: node disjoint and SRLG
From: Stewart Bryant
Date: Fri, 26 Nov 2004 18:30:35 +0000

Joel M. Halpern wrote:

There is a subtlety that I am missing in the phrase 100% protection for some traffic". Do you mean that we should be careful not to redirect (LF, U-turn, or tunnel) traffic which will not be properly protected by the technique, and use the tool for traffic for which we can know it will work?
I had thought that when we said 100% protection we were talking about
proptecting all traffic against all possible failures. Maybe I
misunderstood.
Mike and I wanted to do all src all dest repair for any failure.

Say you cannot do this, you have a problem. If you use uncontrolled
convergence then you damage the repairs, but you restore connectivity
to unrepaired prefixes ASAP.

If you use controlled convergence you protect the repairs, but you
blackhole the unrepaired prefixes whilst you converge - which takes time.

The suggestion below is to run controlled convergence on the prefixes
that are repaired and uncontrolled (faster) convergence on the rest.

Stewart




Yours,
Joel

At 06:23 AM 11/26/2004, Stewart Bryant wrote:

I think I disagree with that argument. Consider a case where one wants to protect VPN traffic, but not necessarily all traffic. In that case, as long as all sources to the set of relevant PE routers are protected, it doesn't matter (as much) that the other destinations aren't protected. There can definitely be cases where one or more particular destination nodes are of the most importance; think of protecting a valuable video broadcast stream, for instance.

What we have not discussed, but which applies in this case, is applying LF to a subset of the prefixes.
What Mike and I have always been alarmed at is the thought that in
protecting some subset of the prefixes, we damage - perhaps quite badly- the service to other prefixes unless we have 100% coverage.
Now what we could perhaps to is to run LF on just the prefixes that
we can protect. So if we knew that we had 100% protection on say
VPN and PWs. We could run controlled convergence on the adresses
of those endpoint address and run uncontrolled on the rest.


Stewart


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