What is the expected iBGP behaviour for routes with different local_pref?

[Ondrej Zajicek]

On Mon, Jul 01, 2024 at 10:26:40AM +0200, Nico Schottelius via Bird-users wrote:
> 
> Hello everyone,
> 
> I was wondering how bird *should* behave if within one AS a route is
> received from outside multiple times but with different local_pref set?
> 
> My expectation would be that using iBGP, the lower local_pref route
> would show up on all routers, but the higher one is selected.
> 
> What I observe is that seems the router who initially receives the lower
> local_pref route does not export it via iBGP, even though it matches the
> export filter.
> 
> ...
> 
> So is this a misconfiguration or a bug or intended behaviour?

Hi

In general, only the best route is exported. The route with higher
local_pref is the best route, it is not exported to other IBGPs due to
being received from an IBGP, but still prevents export of less-prefered
route.

You can enable the option 'secondary' on IBGP and the option 'sorted' on
the routing table, this would change the behavior to 'export the best
route from exportable routes', thus the best EBGP route pasing filter
when exporting to IBGP.

Note that exporting paths that are not best / active in FIB may be
problematic, as it may lead to routing loops, but in this specific
case (propagating best EBGP) it is likely safe enough.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."