Multi-homing with multiple ASNs

William Herrin bill at
Fri Nov 21 10:23:43 UTC 2014

On Thu, Nov 20, 2014 at 5:00 PM, Curtis L. Parish <Curtis.Parish at>
> We have recently added a second ISP  (third if you count I2).
> Our first "ISP" is actually a private state network that peers with
> two Tier 1 providers.  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) so they can
> load balance their connections.    If you hit the right looking glass
> server you can see our network advertised by three different
> ASNs.    We were told by the new ISP that this is a problem but
> the state network says it is not.


If you drop your connection to the state network, do the routes with their
AS numbers drop out of the looking glasses? If not, then there's a problem.

If you depreference your connection to the state network by prepending your
AS number, do comparable prepends appear at the looking glasses or does the
state network continue to give its advertisement of your address space top
billing? If the state network's behavior strips your ability to load
balance your network then there's a problem.

Conventionally, the state network should be adding its AS number after
yours, not stripping your AS number. More often than not, this convention
is also the technically correct course of action.

Bill Herrin

William Herrin ................ herrin at  bill at
Owner, Dirtside Systems ......... Web: <>
May I solve your unusual networking challenges?

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