[Xorp-users] Avoiding asynchronous routing with OSPF

[James Courtier-Dutton]

2008/9/12 Dirk H. Schulz <dirk.schulz at kinzesberg.de>:
> Hi James,
>
> --On 4. September 2008 16:36:42 +0100 James Courtier-Dutton
> <james.dutton at gmail.com> wrote:
>
>> On 12/08/2008, Dirk H. Schulz <dirk.schulz at kinzesberg.de> wrote:
>>>
>>> The problem I have is that packets from servers in subnet B go out via
>>> router2 and answers partially come back via router1. I tried to avoid
>>> this by setting interface costs:
>>> 1. The hightest interface cost is on eth1 which connects my routers
>>> directly (so data traffic between MY routers should be avoided).
>>> 2. The interface ethA without virtual ip address A (that is ethA on
>>> router2) has a significantly higher cost than the other; with ethB it is
>>> similar.
>>>
>> Do you understand the cost setting in OSPF?
>> The ospf cost set on an interface is the cost to SEND packets out.
>> The ospf cost set on an interface has no affect on incoming packets.
>> I only ask in case you have set the cost on the wrong end of the link.
>
> I did not understand that detail yet, but it does not make any difference in
> my scenario - I have recalculated the path costs with that information.
>
> Seems that my problem is a bit more complicated:
>              -------------                  -------------
>              |      1     |-----------------|      2     |
> upstream routers            |                 |            |
>              |            |                 |            |
>              -------------                  -------------
>                     |                              |
>                     |                              |
>                     |                              |
>              -------------                  -------------
>              |      1     |-----------------|      2     |
> my routers (connected via eth1)               |            |
>              |            |                 |            |
>              -------------                  -------------
>                 |      |                        |      |
>                 |      |                        |      |
>                 |      |     -----------        |      |
>                  -----------|          |--------       |
> switch A (subnet A)           |          |
>                        |    |          |               |
>                        |     -----------               |
>                        |                               |
>                        |     -----------               |
>                        -----|          |---------------|
> switch B (subnet B)           |          |
>                             |          |
>                              -----------
>
> I want packets destined for subnet B always to come in on my router nr. 2
> (and subnet A vice versa). But if a packet for subnet B arrives on upstream
> router 1, that means it has to do one more hop using upstream router 2 to
> reach my router 2 (compared to going via my router 1).
>
> Can I configure OSPF so that path costs are weighed higher than hop count?
>
> Dirk
>

OSPF does not care about hop count. It is purely cost.
Just make the outgoing cost on interface 1 -> B high, and 2 -> A high.
OSPF will then prefer 1 -> 2 -> B over 1 -> B.
and prefer 2 -> 1 -> A over 2 -> A.