OSPF Multiple prcoess-id and Areas 0

[yamaan]

In the phase of assessing a certain enterprise network that running OSPF, It was found that they have three of Area 0s as shown in the attached drawing.

Having this, made an automatic network assessment tool to think that we have dis discontiguous area 0s.

Is it a design mistake to have two OSPF 2s surrounding the OSPF 1? Is there any Cisco document talks about not recommending that?

[lgijssel]

In OSPF, the process ID is not the same as in other routing protocols. It is the area number that decides. Two routers need not have the same process ID but in your situation I agree that you have a discontiguous area0. You will find more info about this in the OSPF design guide at the following URL:

http://www.cisco.com/en/US/partner/tech/tk365/tk480/tsd_technology_support_sub-protocol_home.html

[b-watkins]

At least insofar as the drawing you give is concerned, you do not have discontiguous area 0s. What you have is 3 autonomous systems each with their own area 0.

In situations where your'e trying to merge companies that have their own OSPF setups, then it's possible that this would be a reasonable interim solution to a a potentially sticky problem. However, I do not believe that this is a good long-term design. Even if the organization as a whole were large enough to warrant multiple IGP ASes, I wouldn't personally redistribute them into each other directly. I would probably move to BGP as an inter-AS solution if it were deemed necessary to continue the current design in some fashion.

That said, unless we're talking about an extremely large organization, OSPF is probably more than adequate for the whole of it so long as the implementation is well-designed.

[yamaan]

Actually, the network has 3 OSPF processes on some security purpose.

My point is, If they need to have three OPSF processes through the network, they were to give them three different numbers (e.g OSPF 1,2 and 3.) rather than giving the same number to 2 of them.

What do you think?

[b-watkins]

From the standpoint of being able to easily differentiate between the ASes, then I do think it's a good idea to have different process IDs for them. Insofar as the routers are concerned, they could care less. But from an operational view, I think it makes sense to have 3 different processes numbers.