[quagga-dev 5731] Re: show ip ospf route question

Andrew J. Schorr aschorr at telemetry-investments.com
Fri Aug 15 17:28:58 BST 2008


On Fri, Aug 15, 2008 at 05:43:05PM +0200, Joakim Tjernlund wrote:
> I have ospf enabled on a few ppp links on a none with source
> IP address 192.168.101.77
> 
> When I type show ip ospf route I get
> ============ OSPF network routing table ============
> N    172.16.6.29/32        [10] area: 0.0.0.0
>                            directly attached to p1-3-3-3-4
> N    192.168.101.77/32     [20] area: 0.0.0.0
>                            directly attached to p1-3-3-3-4
>                            directly attached to p1-12-3-12-4
>                            directly attached to p1-11-3-11-4
> N    192.168.101.96/32     [10] area: 0.0.0.0
>                            directly attached to p1-9-3-9-4
> 
> I see that I got a route to myself on 3 ppp interfaces, is this
> normal? My old zebra based router does not show this.

I think this is almost "normal", but not quite.  These sorts
of strange routes have been discussed many times on the list,
for example here:

   http://lists.quagga.net/pipermail/quagga-users/2004-January/001382.html

If you have a router with a PtP interface configured with
local address X/32 and remote peer address Y/32, then quagga
typically sees a route to Y/32 as directly attached,
and an additional route to the local address X/32 via the
remote host (since the remote host is advertising a
route to its peer X/32), with cost set to the sum of
the costs of going over the link to the peer and then
coming back.

Your config seems slightly different in that it is saying that
X/32 is "directly attached" instead of saying "via Y, <ptp iface>".
Is the cost 20 equal to the cost you've assigned to those interfaces,
or is it twice the cost (for a round-trip)?
Are you using vanilla quagga or a version with your unnumbered
PtP patch?

Arguably, in a better world, quagga would see that X/32 is
actually a local address and suppress the route through
the peer host.  In practice, it doesn't seem to affect actual
routing, so nobody has bothered to try to fix this.  A patch
would be nice so that people would stop asking about this...

Regards,
Andy



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