Cool OSPF Problem
I ran into a pretty obscure case with OSPF today and I actually don't know of a good way to fix it. See the topology and problem in the link below.
Basically if fa0/0 was shutdown on R3, then R3 would have two equal cost routes for the default route path. However, since R3 is also now learning the default route from R5 (with an LSA type 5 via area 0), it is now being preferred over the type 7 default from R2 and R4, It means that now R4 takes the path via R6 for routes via the default route.
So I would like to know what design decisions should be normally made to stop a problem like this occuring. It's very interesting because no redistribution occurs in this network at all.
The only way I can think of stoppnig this is by using a distribute list like below, but this really seems like more of a hack than a fix for the design.
ip prefix-list STOP_DEFAULT seq 5 deny 0.0.0.0/0
ip prefix-list STOP_DEFAULT seq 10 permit 0.0.0.0/0 le 32
router ospf 1
distribute-list prefix STOP_DEFAULT i fa0/0