[AusNOG] dual home - set one carrier to secondary with AS prepend - not working

Mon Dec 21 13:34:04 EST 2015

Welcome to the world of BGP multi-homing, simply put! How people decide 
to route traffic TO you, is "ultimately" out of your control. Yes there 
are some tips and tricks (like AS path prepending) that can help, but AS 
Path can be overridden very simply by networks that want to.

  The best bet, and may not be possible for you, is to use bigger and 
smaller advertisements. For instance, if I had a /22 (Which is 4x/24's), 
I'd advertise the 4x /24's via my primary link, and only the /22 via the 
secondary, so the more specific /24 route would more likely take 
precedence in other networks routing tables, then if the primary drops 
they move to using the /22 as the /24's get withdrawn.

  Since you say you have 3x /24's, sounds like they may all be 
stand-alone and this may not be possible for your situation, but 
ultimately if another network wants to send to you via NextGen - short 
of actually not advertising via NextGen, you are not going to be able to 
force them otherwise.

On 21/12/15 13:26, Mitchell, Peter wrote:
> Hi All,
>
> First time post, so I hope this is the appropriate query for this list.
>
> I'm trying to steer incoming traffic to our 3 x /24 networks via Testra as primary carrier.  On secondary link (nextgen) we use multiple AS path prepend on the BGP configuration.
>
> Outgoing routing is fine, however incoming we still have substantial amount of traffic come in secondary link.  (approx. 50%)
>
> Telstra think they are advertising correctly to internet and other carriers are overriding the advertisements based on commercial reasons.
>
> Nextgen doesn't think Telstra are advertising correctly domestically.
>
> Using the looking glass tools, it looks like places like iinet are learning networks only via nextgen.
> Telstra say optus looking glass shows network learnt ok via Telstra.
> Iprimus seems to be the only one that has both, but prefers nextgen advertisement.
> AAPT tracert goes via Telstra.
>
> Thanks
> Peter.
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