input filter to set outgoing prefferences

Ondrej Zajicek santiago at
Thu Jun 10 14:13:27 CEST 2010

On Thu, Jun 10, 2010 at 09:57:59AM +0200, Arjan Filius wrote:
>    Hello bird-users,
>    (email send in html to prevent text wrapping, .txt version attached)
>    running bird 1.2.3 ppa x64 ubuntu 10.04LTS. 2 bird routers connected with
>    2 iBGP sessions over different links.
>    I'm trying to route specific outgoing traffic over a specific Transit.
>    It starts with that I'm not really sure how to do it, so tested a bit.
>    Setting Local performance
>    on protocol definition works just fine for a complete  Transit, however
>    when i want to do it more fine grained, i run into trouble.

You mean local preference (LOCAL_PREF BGP attribute)? You can set local
preference in filters for specific routes like other attributes (using
'bgp_local_pref'). That would probably do what you want.

>    First started with prepending routes in bgp input filter with my own AS,
>    which works OK on a single bird router, however, with iBGP all prepended
>    local AS-es are stripped, and that concept doesn't work.

You can also prepend peer AS on incoming routes. If you keep such routes
in your AS it should be OK.

>    Then, started to set the preference in the bgp input filter, which also
>    works fine on a single bird router, but that preference seems not to
>    propagate to the second iBGP router.

Yes, (protocol) preference is not BGP attribute, but it is internal BIRD
metric to be able to compare routes from different protocols (BGP/OSPF/static

Protocol preference (route attribute 'preference') and BGP local preference
(route attribute 'bgp_local_pref') are different concepts.

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
"To err is human -- to blame it on a computer is even more so."
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