In my previous entry, I have mentioned the methods that can be used to interconnect different areas in OSPF. In this entry, I would post another similar network diagram. with possible methods to connect the Areas to have full network connectivity.
Diagram 1 shows that We have two Area 0 in a network, between them we have Area 1 and Area 2.
A simple solution will be to have two virtual-links between R3 – R1/R2, and R4-R3. Another will be to have two GRE tunnels between R3-R1/R2 and R3-R4. Or mix of both, virtual-link between R3-R1/R2 and GRE tunnel between R4-R3.
One interesting and very easy method is to have two instances of OSPF in R3, and perform mutual redistribution. How it works?
R4 is part of Area 0, while R1/R2 are part of Area 0 as well. What remains in R3, which is not connected directly to Area 0 by any side. But, if we break OSPF instance in two. we end up having two full proper OSPF networks. The right of R3 we have Area 2 connected to Area 0. while the Left of R3 we have Area 1 connected to Area 0. Hence, doing mutual redistribution at R3 between those two OSPF networks will yield a full network database.