NSSA forwarding address

[somnath mani <[e-mail]>]

Folks,
The RFC for NSSA states that we set the forwarding address in a type 7 LSA to be that of any interface on which OSPF is configured ( barring the scenario, when the interface to the other AS is internal to OSPF).
This is a problem when you are trying to do equal cost multipath routing and in some other situations.

Consider a router A with 2 interfaces configured to run OSPF, IF1 and IF2. The cost on both these interfaces is 10 , let's say.

If we pick the forwarding address to be IF2 ( We are redistributing at router A), then a router B adjacent to A over IF1 will compute the cost for a type 7 LSA route to be 10 + 10 = 20 ( Assume either ends of the interfaces are configured with same cost). Whereas, router C, adjacent to router A over IF2 will compute the same to be 10 !!

Routers upstream of Router B and C will prefer to route via C !

Would it not be better if we set the forwarding address to be the router ID ( provided we have added it as a stub host ) ?

Thanks
Somnath




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