In message <[email protected]>
Alia Atlas writes:
> 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
There is still no possibility of routing loops in multiarea TE.
Actually it is a benefit of source specified forwarding. So something
like NIMROD would have the same benefit even though it is
significantly different from MPLS RSVP/TE. NIMROD is not an overlay.
> >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.
Again - terminology. Uncorrelated failures would be ones that don't
happen at the sake time. I still got what you said.
Yes the tradeoff is probably worth it. The point that I was trying to
make is you *can't* loop with MPLS/TE and you *can* loop with IP even
without IP FRR. Looping is really bad since you affect other triaffic
if you congest links going around long enough for TTL to expire.
> >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.
Its nice to be in agreement. A real good question for providers
though is given the widespread use of TDM or switched services in low
speed, and WDM in high speed networking, is IPFRR useful without SRLG.
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