Extending a Large OSPF Stub Area

[Garry Cooper]

Currently we have a large ospf stub area, this is conencted to Area 0.

We need to extend one of our sites and add another Area to it..

I have tried making the site an nssa but this doesnt work.

Plus I dont have the option of changing this area to a normal ospf area, as its to large to change.

What is the best practice to extend this network.

Any info woud be mucg appreciated.

[Peter Paluch]

Hello Garry,

You're trying to do something OSPF does not want you to do, for a good purpose - a well-designed network should not chain areas in OSPF but rather have all non-backbone areas directly connected to Area 0.

If your existing "stub" area was actually a normal area you could at least configure a virtual link that would allow adding the new area in a daisy-chain fashion. However, a virtual link cannot be configured over a stub area.

If you absolutely insist on having a new area and not just extending the one you currently have then probably the easiest way of accomplishing this is by configuring a GRE tunnel that is terminated on an ABR between the backbone and the "stub" area, and the other end is on the ABR between the "stub" area and the "new stub" area, very similar to how you would configure a virtual link. This GRE tunnel shall be put into OSPF area 0 and OSPF shall be run over it. This will allow your newly added area to both access the backbone (via tunneling) and the existing area (as the ABR will be able to allow the exchange of inter-area routes between the old and new stub areas).

Best regards,

Peter