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OSPF - Multiple ABR's

I've a topology where I've remote locations with frame circuits back to a central facility with redudant infrastructures. The central facility has two 7200, one with dual circuits to AT&T and another with dual circuits to MCI. The remote facilities have the same arrangement. Each location also has a connection to the internet through a third router.

We currently run rip and static routing and want to move to OSPF.

What types of areas would the remote offices be considered in this arrangement?

Also, does it make more sense to put yet another router at each facility to function as the default gateway of for the office and have it receive a summary route from the two edge routers?

Thanks,

Bob Pierce, ProBusiness

bpierce@probusiness.com

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New Member

Re: OSPF - Multiple ABR's

You can have a) everything in area 0, b) all frame relay routers in some single non-zero area (attaching to area 0 only at the central site), or c) you can make your frame relay routers at all sites (remote and central) ABRs attaching to area 0 towards the center and their respective area out from there. The latter of those three would probably be the best choice in most cases. There are some drawbacks though. All summary advertisements have to traverse the frame relay cloud for one thing (because it is area 0).

As for what types of areas your remote sites will be (assuming you went with option c), I would guess they would need to be set up as NSSAs because of your internet connectivity at each site. An NSSA behaves like a standard stub area in that it doesn't allow type 5 (external) LSAs into the area via the ABR. However, your internet routers become ASBRs and inject type 7 (special external) LSAs that are either restricted to that area or converted to standard type 5 LSAs by the ABR prior to injection into the backbone.

You can still summarize network advertisements coming in from the backbone at the ABRs. And you can have ASBR routers at your central site distribute a default route out to all of the remote sites too.

The other thing that you have to think carefully about is your NBMA configuration for OSPF. This is largely dictated by whether you went with mutipoint subinterfaces or point-to-point subinterfaces.

Finally, before making any commitments, you may want to evaluate the stability of your networks overall. You can end up with a lot of link state database activity when you mix OSPF and frame relay if your not careful.

Check out this link:

http://www.cisco.com/cgi-bin/Support/PSP/psp_view.pl?p=Internetworking:OSPF

This is actually a link to many, many more links. I would recommend, at a minimum, that you read the OSPF Design Guide.

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