bgp route selection

[Ondrej Zajicek]

On Tue, Apr 08, 2014 at 07:11:36PM +0200, Frederik Kriewitz wrote:
> On Tue, Apr 8, 2014 at 5:50 PM, Thomas Goldberg <t.goldberg77 at gmail.com> wrote:
> > Is there any other solution for this besides doubling the iBGP
> > connections (adding seperate blackhole only sessions with higher
> > preference)?
> 
> We faced the same problem in the past.
> The preference can be set on a per route basis during the import.
> If you add the following line to your import filter it should do the
> trick for you:
> preference = bgp_local_pref;
> That way the bgp local preference is used as the bird internal preference too.

That will work, but in this case i would prefer an alternative: do not
depend on recursive resolution of bgp_next_hop to blackhole route, but
set blackhole destination directly in filters:

if bgp_next_hop = A.B.C.D then {
  dest = RTD_BLACKHOLE;
}

In that case recursive resolution of next hop is not used and
resolvability test does not apply.


> Having read only the mentioned section of RFC 4271 someone might argue
> that bird should consider blackhole routes as resolvable too:
> 
> ...
>
> That's how it's implemented on our alcatel/cisco/juniper routers too,
> bird seems to be the exception here.

Well, it is debatable whether IP covered by unreachable/blackhole/prohibit
routes should be considered resolvable for these purposes. I would say
that semantically such addresses are more-or-less equivalent to
addresses not covered by any routes.

> Overall I'm a little confused about the implementation now too. If the
> next hop is not resolvable it shouldn't be concidered at all but bird
> apparently still considers blackhole routes as a last resort or with
> higher bird preference.

That is true, BIRD currently cannot completely suppress a route based on
unreachability of the nexthop, it just make unresolvable route
a lowest priority route in its preference group.

-- 
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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