route withdraws on BGP route-refresh

[Ondrej Zajicek]

On Mon, Dec 28, 2015 at 10:41:26AM +0200, Christos Trochalakis wrote:
> On Thu, Dec 10, 2015 at 12:10:35PM +0200, Christos Trochalakis wrote:
> >Hello all,
> >
> >While debugging a route-refresh issue we discovered that when a route-refresh
> >is received, bird, instead of just sending an update containing the exported
> >routes, also withdraws **all known routes** from its peer. In our case, it
> >withdraws the whole FIRT sending multiple BGP updates. Is this the expected
> >behaviour? We are using bird 1.4.5.
> >
> >We believe that the opposite router has trouble processing all those withdrawed
> >routes which leads to other connectivity issues.
> >
> 
> Any comments on that?
> 
> Let me restate the issue, bird withdraws every route in its routing
> table despite the fact that those routes were never exported to that
> protocol. I am not familiar with bird internals so I don't know if bird
> keeps track of the exported routes per protocol.

Hi

BIRD does not explicitly keep track of the exported routes per protocol.
In normal operation, it just evaluates filters on both the old and the
new route to see if it is necessary to send an update - if both are
rejected, then no withdraw is sent. But this approach is not used for
full table refeed (which may be triggered by reconfiguration, explicit
command or received route-refresh). In refeed, every available route is
either propagated or withdrawn, mainly to handle situation where export
filters were changed and therefore we cannot reliably decide whether
routes were exported in the past.

-- 
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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