Wired Bird BGP behaviour

Ondrej Zajicek santiago at crfreenet.org
Tue Sep 22 09:15:40 CEST 2009

On Mon, Sep 21, 2009 at 11:04:36AM +0200, Ondrej Zajicek wrote:
> On Mon, Sep 21, 2009 at 10:10:43AM +0200, Dr. Christian Riede wrote:
> > On Test-Quagga1, I see only 3 accepted prefixes on the sessions to Bird
> > (the direct attached networks).  When I shut down all eBGP sessions on
> > Test-Quagga1, I get the expected ~290000 accepted prefixes on the two
> > sessions to the Bird. The routing table grows very slowly.
> > 
> > If I stop Bird and start Quagga on the same system, I see all expected
> > ~290000 prefixes on Test-Quagga1 via iBGP.
> > 
> > It seems that Bird does not re-announce routes to Test-Quagga1 that it
> > learnt via eBGP if it also receives those routes from Test-Quagga1 via
> > iBGP. This behaviour is different to Quagga.
> > 
> > Is my suspicion correct?
> > 
> > Can anyone explain this behaviour?
> Perhaps BIRD learned the prefixes from Test-Quagga and prefer that route?
> In that case, it shouldn't propagate less prefered routes received
> through Quagga1.

Yes, it is this behavior and it is probably caused by different default
value of LOCAL_PREF attribute. This attribute is expected to be
configured consistently on all routers in an AS accorrding
to a local policy. But it is not specified what is a default value -
BIRD uses 0 and Quagga uses 100. Therefore your Bird prefer
prefixes received from Test-Quagga as it expected that they come
from preferred uplink eBGP.

Therefore, you should configure BIRD and Quagga to use the
same LOCAL_PREF value for the same eBGP uplink.

In BIRD, it should be done using import filters in eBGP protocol:
import filter { bgp_local_pref = 100; accept; };

There is also option default_local_pref, it does something different
(and is mostly useless). 

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at 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."

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