[Xorp-users] troubleshooting wrong route in OSPF?

[Alain Fauconnet]

Hello readers,

Sorry if this sounds a bit like a newbie question, but I've browsed
the available documentation and couldn't find any answer to help me
diagnose this (like for tracing OSPF advertisements, costs)

I have a case of XORP's OSPF installing the wrong route in a simple,
single area (0.0.0.0) configuration:


    a.b.63.4/30   .5 a.b.36.0/24 .1
[4000] ----- [XORP] ------+------[7200] a.b.128.2
     .5     .6            | .10
                       [Zebra]

Both the XORP and Zebra boxes are Linux-based, the other boxes
are a 7200-series router and a Catalyst 4000-series L3 switch
(which doesn't seem to have any relevance here, but just 
to be complete...)
a.b.128.2 is a loopback interface on the 7200.
As showem, 3 boxes have an interface in network a.b.36.0/24:
Xorp: eth0 is a.b.36.5
Zebra: eth0 is a.b.36.10
7200: FastEthernet2/1 is a.b.36.1

On the XORP box:

> ip route get a.b.128.2
a.b.128.2 via a.b.36.10 dev eth0  src a.b.36.5 
    cache  mtu 1500 advmss 1460

which is obviously wrong.
Both neighbors are up though:

> xorpsh -c 'show ospf4 neighbor'
Address      Interface              State     ID             Pri  Dead
a.b.36.10    eth0/eth0              Full      a.b.36.10      1    33
a.b.36.1     eth0/eth0              Full      a.b.128.8      1    38
a.b.63.5     eth1/eth1              Full      a.b.128.6      1    36

So the (2 hops) route advertised by the Zebra box is preferred to the
1 hop route to the Cisco router... how can this be?
Where can I go from here to troubleshoot this? I would know on a Cisco box
(looking at costs etc.) but I feel helpless with Xorp.

Thanks in advance for your time reading this and any tip, even RTFM links.

Greets,
_Alain_