On Sun, Mar 28, 2010 at 10:19:59PM +0100, David J Craigon wrote:

Hello,

Here's what I want to do. I've got bird running BGP to two transit network providers. I want my path selection to be like this:

1) Choose the shortest AS path. 2) If the AS path is equal in length, prioritise one provider (the cheaper one!) over the other.

Possible, or not? I can do it with local priority, but then all traffic goes down one provider. I want to supply the tiebreaker when it comes to equal length AS paths.

There are several ways to do that, but they are not straightforward. One possibility is to abuse origin attribute for this purpose. Comparison of origin attribute is used as a tiebreaker after AS path length comparison. So you could modify origin attribute in import filter (all routes from the first provider get one value, all routes from the second provider get another value). But you should not export such modified routes to anybody (as origin attribute is not expected to be modified), so this way is possible only for non-transit ASes. But i would suggest something different - just prepend one AS number to AS path of less prefered provider in import filter. This would lead to slightly different, but similar behavior. Advantage of this approach is that you can do it symmetrically in other direction (prepending your AS number in export filter) and alter incoming traffic and not just outgoing traffic. -- Elen sila lumenn' omentielvo Ondrej 'SanTiago' Zajicek (email: santiago@crfreenet.org) OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net) "To err is human -- to blame it on a computer is even more so."