Why doesn't BGP...

Sat Nov 9 03:23:17 UTC 1996

Ed Morin <edm at halcyon.com> writes

 * We peer, using BGP, with several "backbone" provider networks for transit
 * purposes.  Some of these links are "faster" than others (e.g. T-3 vs.
 * multiple T-1 and single T-1) for various reasons.  If our router sees
 * a route to a particular destination via a "high-speed" link and a "low-
 * speed" link that has the _same_ number of AS "hops", it picks the link
 * with the "lowest" IP address!  (At least that's what I'm told and what
 * I observe...)

Yup, this is the ultimate tie breaker if there are two routes that are
otherwise identical.

 * Why doesn't BGP pick the link with the highest bandwidth, or, better
 * yet, pick the link with the highest bandwidth AND least congestion to
 * label as the "best" available route?  The needed information is avail-

The first one is easy, in fact you can do that yourself by fiddling
with metrics or such on the different BGP sessions. The second one
would have dramatic consequences in terms of route instability. You
pick one route now because of load on the link, the load changes and
you pick the other, now BGP will have to change the announcement of
this network to other peers. So, now we not only have flaps because of
links/routers going up and down, we also have flap because of load
changes on the network. The result: you are dampened out forever, or
the network falls over.

-Marten