Unreachable next-hop over bgp

[Ondrej Zajicek]

On Wed, Aug 17, 2011 at 06:45:24PM +0100, Alex Bligh wrote:
>> The only reason for this I can see is because bird is determining
>> reachability on the basis of its FIB only (as opposed to the kernel
>> routing table) and there is no FIB entry for 10.255.199.0/24
>> (from show route).

Yes, that is exactly the reason.

>> How are such interface routes meant to get into the FIB?

Short answer is to use 'direct' protocol to populate BIRD routing table
by device routes.


See the discussion in FAQ
(in 'IBGP does not work after upgrade to BIRD 1.3'):
https://git.nic.cz/redmine/projects/bird/wiki/FAQ


> 2 further bits of information:
>
> 1. bird version 1.3.2-1 (Ubuntu Oneiric)
>
> 2. gateway-direct fixes this, but I don't understand why. If a direct
>   lookup succeeds, then surely a recursive lookup must always succeed?

No, these are two different mechanisms:

- recursive lookup tracks BIRD routing table.

- direct lookup just examines interface address ranges.

And there are more minor differences (like recursive mode uses just
the NEXT_HOP address, while direct mode fallbacks to BGP peer address
if address from NEXT_HOP is not a neighbor)

-- 
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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