On Jan 31, 2009, at 10:17 PM, Enke Chen wrote:
So here is an alternative (operationally):
accept MEDs from each other and perform longest-exit.
The MEDs could be set by the sender based on the location (community)
so that at most 2 or 3 peering locations would advertise the same
MED.
For example, for a particular prefix originated from New York:
Peering Location MED
-----------------------------
New York / DC 10
Chicago/ Denver 20
SFO / LA 30
.....
The location/community based route-map should be uniform (i.e.,
identical) at the peering
locations.
This approach should reduce the number paths significantly, and yet
maintain redundancy
and convergence properties in a network.
The increasing number of interconnections will still be
necessary, and the number of internally amplified routes
will grow accordingly. With each BGP speaker or Route
Reflector set/cluster MEDs would possibly decrease the
number of routes contained in the core (as every router
wouldn't default to advertising the local paths as best)
but it only marginally improves the problem, which is still
highly topologically dependent and constrained by the
current BGP internal scaling model.
Furthermore, MEDs trigger lots of other badness (e.g.,
oscillation), require inter-provider coordination and
normalization (e.g., with different IGPs and metric space,
internal metric allocation models and multipliers vary
considerably, etc..), can make peer gaming much simpler,
and are usually broken when considering routes originated
in multiple locations from an adjacent peer AS.
I.e., MEDs should turn closest-exit (hot potato) routing
into best-exit routing (cold potato/?), but in my experience
most often result in mashed potato routing (e.g., S 7.1.1
of RFC 4277).
-danny
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