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Re: questions on draft-bryant-ipfrr-tunnels-01.txt

Subject: Re: questions on draft-bryant-ipfrr-tunnels-01.txt
From: Alia Atlas
Date: Mon, 22 Nov 2004 13:29:55 -0500
At 10:16 AM 11/19/2004, Curtis Villamizar wrote:

MPLS RSVP/TE FRR is loop free in the presense of unknown SRLG.  At
worst the fast restoration fails and a new LSP is needed from ingress
to egress.  So the worst case is a slow restoration but not a loop.
This is true, but only for the length of the TE tunnel. TE gets this
benefit only because it is an overlay approach; the issue occurs where ever
the overlay ends. If one is doing 1 or 2-hop tunnels, then looping is also
If you meant all IP FRR can be subject to transient looping due to
unknown SRLG, then you are correct.  That is a consequence of
hop-by-hop forwarding.  IP itself is subject to transient looping due
to correlated failures even without IP FRR.  We seemed to have lived
with it so far.
Part of the question is how often do uncorrelated failures happen as
compared to single failures (which could also be a known SRLG)? If
uncorrelated failures are relatively rare in comparison, then the trade-off
of saving the traffic via an IPFRR alternate for the single failures in
exchange for potentially looping saved traffic for an uncorrelated failure
seems reasonable to me.
I don't know that the WG has decided that SRLG is a MUST.  It should
be obvious from what I've written that I think its important.
I also think that SRLGs are important. We do need to have the basic
mechanisms understood before trying to add the additional complexity of
SRLG handling. I certainly think that the ability to handle SRLGs is an
important criterion in considering different approaches.

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