I am trying to determine the optimal Bidirectional Forwarding Detection
(BFD) settings for BGP auto-discovery and layer-2 signaling in a VPLS
application.
To simplify things, assume I am running LDP for building dynamic-only
LSPs, as opposed to RSVP. Assume I am running IS-IS as the IGP with BFD
enabled on that, too, interconnecting all of the P and PE routers in the
MPLS cloud. I am following the Juniper recommendation of 300 ms mininum
interval with 3 misses before calling a BFD down event.
The network design has a small set of core routers, each one of these
routers serves as a BGP route reflector. All of the core routers have
inter-meshed connections. Each core router is only one hop away from the
other.
On the periphery, I have perhaps dozens of distribution routers. Each
distribution router is directly connected to two or more core routers.
Each distribution router has a BGP session to these core routers;
therefore, each distribution router is connected to two route reflectors
for redundancy.
Given that above, what type of minimum interval BFD setting and miss count
would you configure? I want to try to get to a sub-second convergence
during node/link failure, but I do not want to tune BFD too tight and
potentially introduce unecessary flapping. I am willing to suffer some
sporadic loss to the layer-2 connectivity within the VPLS cloud in the
event of a catastrophe, etc, for a few seconds, but I don't want to
unnecessarily tear down BGP sessions and wait some 20 to 60 seconds or so
until BGP rebuilds and redistributes L2 information.
For some time now, I have been playing with 3000 ms interval with 3 misses
(that's 9 seconds) as what I thought was a conservative estimate.
However, I have run into cases where there has been enough router churn
for various reasons to uneccesarily trip a BFD down event. My hunch is
that this "router churn" is due to buggy JUNOS code, but I don't have
proof of that yet. Nevertheless, I want the BGP infrastructure to stay
solid and ride through transient events in a redundant network.
Are there any factors that I am missing or not thinking thoroughly enough
about when considering optimal BFD settings?
Thanks.
Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
Williamsburg VA 23187
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