anycasting behind different ASNs?

James Jun james at
Wed Dec 6 22:17:23 UTC 2006

> My real question was whether anyone actually cared about inconsistent AS
> paths and did something (drop traffic) because of it.  Appears that it's
> not much of an issue.

Not any significantly sized AS at least..  IMO, filtering inconsistent AS
would be more "too much time on hands" and hectic process than building and
updating prefix-list to drop top 10 deaggregators listed in
:)  In operational sense, there should not be any problems in reaching
almost the entire internet in my experience.

There are some issues with incon. AS_PATHs as others noted, but technically
speaking, BGP in itself doesn't care about it in its route selection
process.  It cares about AS_PATH length (and AS loop) during its selection
process, not what number ends in a path.  This is a common confusion by many
low/mid-level engineers and sales engineers who claim their network having
lower AS number somehow makes them more trafficked and preferred AS :)

But what Steve suggested is probably more important operationally for what
you're trying to do.  Transit/peer/customer relationships often catch
first-timers doing anycast by surprise when routes don't go where they want
to go in some cases.


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