Two backbone OSPF areas...

[dnewell24]

Our enterprise is preparing for network transitions. One approach presented is a new backbone OSPF area 0 connected to the existing non-backbone areas. In theory, as we transition users over to the new backbone they should follow the routes, as they appear, over the new backbone.

I know the above statement was fairly vague but that was the explanation I received. My only question is can two backbone areas exist in the same OSPF ?autonomous? network? In the scenario presented they will connect to the same non-backbone areas but not to each other.

TIA,

Ryan

[Richard Burts]

Ryan

The approach that you describe violates one of the concepts of OSPF which is that the area 0 backbone should be contiguous. It may work or things may break depending on some details of how it gets implemented. Some of the things that can cause black holing of traffic or other problems can include:

- if you have configured stub areas, the stub area will have a default route pointed to its area 0. Traffic to destinations reached through the other area 0 will be unreachable if the area 0s are discontiguous.

- if you have configured summarization it may become ambiguous where some of the routes are located and how to get to them.

- if you configure area(s) as NSSA it may become ambiguous how to get to some destinations.

Without knowing more about your situation it is impossible to know if any of them apply to you. There are probably some other problematic aspects but these are what occur to me first. I would suggest that it would be better if the area 0s could be made contiguous. In the worst case I would suggest consideration of virtual links as a way to make the backbones connected if there is not a good way to accomplish this physically.

HTH

Rick