bgp route selection

[Ondrej Zajicek]

On Mon, Apr 07, 2014 at 05:28:24PM +0200, Thomas Goldberg wrote:
> Hello,
> 
> can yomeone please explain me the following behaviour:
> There's one eBGP session (b_dtag1) and one iBGP session with one of
> our RR Servers (b_rr1).
> The RR redistributes the blackhole prefixes learned from the Spamhouse
> BGP Feed (BGPf).
> 
> The prefix 64.112.128.0/18 is supposed to be blackholed.
> 
> These are the two routes learned from the two peers:
> bird> show route all 64.112.128.0/18
> 64.112.128.0/18    via X.X.X.X on eth0 [b_dtag1 Apr06] * (100) [AS3320i]
> ...
>                    blackhole [b_rr1 15:36 from Y.Y.Y.Y] (100/-) [AS65535i]
> ...
> 
> As you can see the (older) regular eBGP route is picked over the
> blackhole route:
> 
> Based on the Route selection rules from
> http://bird.network.cz/?get_doc&f=bird-6.html#ss6.2 I expected that
> the first (Prefer route with the highest Local Preference attribute)
> and the second (Prefer route with the shortest AS path) rule would
> result in the blackhole route being picked.

Hello

That is because only resolvable routes are considered (RFC 4271 9.1.2.1),
main reason for that condition is to eliminate routes whose bgp_next_hop
cannot be resolved using IGP, it also affects routes whose bgp_next_hop
is resolved to a unreachable/blackhole/prohibit route.

In your case, it could be fixed by adding higher protocol preference
(different attribute than bgp_local_pref, currently 100) to b_rr1.

-- 
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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