Multi-homing with multiple ASNs

William Waites ww at
Fri Nov 21 10:06:38 UTC 2014

On Fri, 21 Nov 2014 11:07:49 +0200, Mark Tinka <mark.tinka at> said:

    >>  We own an AS number and our IP space but at the last minute
    >> learned our state network is advertising our network using two
    >> different ASNs (neither ours)

This will work, as in the BGP path selection algorithm will work as
designed in this situation. But it also means that the routing policy
is out of your control which is kind of the point of having an ASN! It
also makes it harder to track down who is operationally responsible
for that address space since it appears to the outside world to be in
two (or three! different places). I'd say don't do this unless you
really have no choice.

    > Why aren't you originating your own prefixes and ASN by
    > yourselves, since you own both?

Good question.

We (AS60241) almost ended up doing similarly for a while. Because of a
close association with the universities in Scotland, we discussed the
possibility of transit via JANET. This turned out to be difficult
because they run a whole bunch of private ASNs internally -- unlike in
North America where universities typically have their own real one. So
it would have been us -> private stuff -> AS786 and for some reason
that I forget they were unable to remove private ASNs from the
path. The best that might have been possible would be to have had them
announce our networks with synchronisation on, which would have meant
the outside world would have seen them originating in both AS786 and
AS60241. Icky. We (mutually) decided against this.

Just to say that there are strange, but not completely unreasonable
circumstances in which this can happen...

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