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

Mark Newton newton at atdot.dotat.org
Tue Dec 22 15:16:27 EST 2015

Hi Peter.

On Dec 21, 2015, at 1:26 PM, Mitchell, Peter <Peter.Mitchell at santos.com> wrote:
> 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.

BGP doesn’t support the notion of a “primary” and “secondary” link.

You can influence it, but the bestpath to get to you is ultimately chosen by virtually everyone except you. If you’re multihomed, you need to expect that a portion of your traffic will arrive on each link. You can influence the proportional distribution of traffic to an extent, but you can’t say, “This is a backup link so I only want to use it if the primary link is down,” because they aren’t concepts that BGP understands.

As others have said, you can prepend on Nextgen and announce more-specifics on Telstra, but at the end of the day almost all of the ISPs in Australia will be routing to you via Nextgen because they probably peer with them, and can reach you via peering without having to pay extra for it.

Maybe Santos should be peering as well, then you could pick up the same traffic from peers without having to pay Nextgen to deliver it to you.

  - mark

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