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split ospf area

This is not something I'd do in real life but it came up in a lab I was working on.

Two routers are configured in the same ospf area but they are on different vlans and ip networks. A third router on the same switch has a trunk. The requirement is to configure the third router so the first two can see each other.

The obvious thing is to turn on ospf in it but I think I'm suppossed to find another way. I tried bridging figuring that would let them see each others multicast traffic but I think think ip addresses are used to form the adjacency.

It did not work.

Anyone got any ideas???


Re: split ospf area

I would thing that you are to configure the trunk with a subinterface for both the VLANS and enable OSPF on them. Both the other routers will then form an adjacancy with this router and be able to route traffic to the other.

If you do want to bridge the OSPF traffic on the trunk router, then you need to turn off IP routing completely, ie 'no ip routing', or use IRB, Integrated Bridging and Routing, which lets you route traffic on some interfaces and bridge it on others, otherwise the OSPF traffic will be dropped.


Re: split ospf area

We (study group) turned on ospf just to keep going, but the lab specified ospf in a list of routers and this was not one of them. The directions were to make the two ospf routers "see each other" through the other routers. I thought there might be some trick in the router on a stick design but I cant figure out how it would work without ospf in the middle router.

New Member

Re: split ospf area


I have tested the configuration and figured out an IP-in-IP tunnel with separate IP subnet. Both routers made adjacency over this tunnel, when added to area 0.

I hope this was allowed at the lab.