The MPLS WG Archive

[Kireeti Kompella <kireeti@juniper.net>]

Hi Glen, Neil,

> > What is the defect entry criteria on these rapid Hellos?  I would strongly
> > suggest that if anyone is thinking of trying to get very fast reactions
> > times to be used for restoration they should be very careful.  In my view
> > restoration times above the L1 (ie PDH/SDH/OTN) should not be faster than
> > somewhere in the region of 2-3s.....otherwise you are going to get a lot of
> > unecessary hits when the event would of have self-healed.  And if anyone is
> > using some multi-level priority bumping scheme then (esp at highish
> > utilisations) this can lead to significant network disruptions felt far
> > beyoind the initial event source.

Two related points: first, if an LSP (for whatever reason) runs over
unprotected links (say GigE), you would want it to recover as fast as
possible, because L1 just cannot recover.

Second, a unified control plane and a common view of the topology is
needed to make these kinds of decisions.  Say the above LSP runs over
4 segments, two of which are SONET rings, one of which has link local
protection (a la LMP) and the last is unprotected.  The ideal solution
is to protect the last segment with MPLS fast reroute, and to leave
recovery on the other segments to L1 mechanisms.  But to get such fine
grain protection schemes, and to *not* get the "unnecessary hits" from
double protection (as well as the network inefficiency) requires that
MPLS knows what L1 is doing.

I agree with the high order bit, though: having fast protection at
multiple layers is asking for trouble.

> One of the reasons for improving the reaction time, and the OAM performance
> in general, is to dispense with protection at the SDH layer and move protection
> into MPLS.

Absolutely.

> MPLS also allows a wider range of topologies -- SDH is quite limited and
> can't implement all the desirable layer two topologies without having
> huge amounts of idle bandwidth.

Also a wider selection of L1/L2 (what if some of the hops were GigE?).

On a different note:

> NH=> Voice is one application that certainly does not need a sub-1s
> restoration time.

Don't know about this.  I do know that we (and other vendors) have
a lot of customers asking us for sub-second (even 50ms) MPLS fast
reroute.  Even if (big if) voice doesn't need sub-second recovery,
there will be applications that do, and many find the option to use
MPLS fast reroute over GigE or unprotected SONET/SDH gear appealing.

Kireeti.