[c-nsp] OSPF router gets separated from a broadcast domain

[Gabor Ivanszky]

Hi Whisper,

thanks for your response.

probably I didn't explain the issue very clear, I apologize.

I try to give an other example for the same issue, which is a bit more 
practical (and more complicated).

On the attached figure you see a topology which is intended to provide a 
redundant access for NET Y to the Backbone. Routers A, B and C connected 
through a common broadcast multiaccess network. Each of them has 1 
interface in this network and has an IP address from network x.x.x.0/24 
. They are running OSPF on this network, so they are electing a DR, and 
everything works fine until now.

Now the link between the two switches goes down, so the broadcast domain 
is separated into two parts: one part with router A and C, and one part 
with only router B.
A & C elect the new DR. BUT router B can not see any OSPF neighbors in 
network x.x.x/24 anymore, so it became DR too, and starts to announce 
network x.x.x/24. If router B's announcement is more attractive than 
router A's, than probably it will cause a blackhole, and NET Y became 
unreachable.

I could be wrong, because I don't know OSPF path calculation algorithm 
in detail, but I assume it is somehing like this:

NET Y is attached to routerC -> routerC is attached to  x.x.x/24 -> 
x.x.x/24 is attached to router A and router B => if router B's metric is 
better, then we have a blackhole

probably I am overlooking something really simple...

many thanks for your time
cheers,
Gabor