OSPF route selection

[Ondrej Zajicek]

On Tue, Oct 07, 2014 at 06:14:15PM +0200, Thomas Goldberg wrote:
> Hello,
> 
> we're using OSPF for anycast routing and came across a small problem.
> 
> We've a BIRD v1.4.4 router which sits in area 0 and 16.
> Here's the relevant state information (10.236.144.144/32 is announced
> from two different routers in two areas):
> 
> area 0.0.0.0
>         router 10.236.144.241
>                 distance 33
>                 external 10.236.144.144/32 metric 1
> 
> area 0.0.0.16
>         router 10.17.224.253
>                 distance 111
>                 external 10.236.144.144/32 metric 1
> 
> The route via area 16 is considered best:
> 10.236.144.144/32 via 10.176.3.29 on vlan542 [o_internal 2014-10-07
> 16:49:56] * E1 (150/112) [10.17.224.253]
> 
> Given that both E1 routes are reachable via intra-area paths.
> We're wondering why the path via area 16 (higher costs than area 0) is chosen.
> Is this the intended behavior?

Surprisingly yes, see RFC 2328 16.4. (6c) and 16.4.1.:

        16.4.1.  External path preferences

            ...

            The path preference rules, stated from highest to lowest
            preference, are as follows. Note that as a result of these
            rules, there may still be multiple paths of the highest
            preference. In this case, the path to use must be determined
            based on cost, as described in Section 16.4.

            o   Intra-area paths using non-backbone areas are always the
                most preferred.

            o   The other paths, intra-area backbone paths and inter-
                area paths, are of equal preference.


> Is there any way to make bird prefer routes via a specific/backbone
> area in a situation like this?

You could enable 'RFC1583Compatibility' option, but that is generally
discouraged unless you know exactly what you are doing (as it may lead
to persistent routing loops).

Or you could use external routes with different E2 metric, in that case
lower E2 metric is always preferred.

-- 
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."
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: Digital signature
URL: <http://trubka.network.cz/pipermail/bird-users/attachments/20141008/20e4fe72/attachment-0001.asc>