[c-nsp] BFD alternative

[Oliver Boehmer (oboehmer)]

> We're in the the process of turning up an MPLS network using ASR9ks
and
> ME3600s.  We're looking to get away from L2 and interconnect all the
devices
> at L3.  To facilitate this, we were originally going to use unnumbered
on
> all the PE-PE, P-P, P-PE links but we just recently discovered that
BFD
> isn't supported on unnumbered Gig/TenGig interfaces.
> 
> We're just doing L2/L3VPN here.  No VPLS, TE, etc at this point.  That
said,
> if we tune down our SPF timers (ISIS) and enable LDP IGP Sync and LDP
> session protection, is that a pretty good compromise?
 
well, I wouldn't call it a compromise.. BFD addresses failure detection,
and tuning down timers speeds up reaction to failures (once detected).
If it takes 30 sec to detect a failed neighbor (without BFD and using
default hello timers), I would argue that it doesn't matter if ISIS
takes another 5 secs (default timers) or < 1 sec (tuned timers) to
update routing table. So tuning down hello timers would be an
"alternative", albeit not a good one.

If you want to be serious with fast convergence, I would not run it
without BFD, and hence would not use unnumbered links..

	oli