Multi-homing with multiple ASNs

Curtis L. Parish Curtis.Parish at
Fri Nov 21 14:49:17 UTC 2014

Thanks for all the responses.  I will answer a few questions that have come on and off list.   (Sorry for length)

We advertise our ASN into the state network with more specific routes that we advertise via ISP2 via our ASN.    This is done because the state (vendor managed) network runs stateful firewalls and we have to force other multi-home entities on the state network to use our state connection instead of ISP2.   Our network has been removed from the state firewall due to previous problems with asymmetric routing with our I2 circuit.    I am told the state network does drop our network from their advertisements when our network is unreachable.  That has not been explained or tested.

What we did not realize until about a week before turning up ISP2 was the state was consolidating all state networks to use two of the vendor’s ASNs when it peers with their two ISPs.  Our ASN is not part of the path.    We had no choice but to turn up ISP2 due to bandwidth reasons.     Miraculously we achieved almost a 50/50 balance of traffic.    Bandwidth will be increased on ISP2 as demand grows so we will need the ability to prepend on the state network to make ISP2 look more desirable.

I believe the state will modify their advertisements to add our ASN to the path but changes to advertising via the state network has to go through a design and change management process and then be scheduled into maintenance windows.    Any attempts to balance the traffic via prepending will take weeks.    As long as the traffic stays balanced we are OK.    When replaying BGP route changes I normally see our network only advertised out one of state ASNs but occasionally I see it with two so traffic balance may be impacted depending on which ISP the state is egressing.

Here is a question.   I know that having one network advertised by multiple ASNs is unconventional and thus it will probably be harder to get help troubleshooting routing problems when they arise.    Do you see a situation where our network might be caught in a loop or black hole due to asymmetric routing and conflicting advertisements?

Thanks again. New to the list but have already learned much by reading the archives.


Curtis Parish
Senior Network Engineer
Middle Tennessee State University

Subject: Re: Multi-homing with multiple ASNs
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.

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