OSPF Network Topology
OSPF works best in a hierarchical routing environment. When designing an OSPF network, the first and most important task is to determine which routers and links are to be included in the backbone (area 0) and which are to be included in each area. The following are three important characteristics to OSPF to ensure that your OSPF network has a hierarchical routing structure:
The hierarchical routing structure must exist or be created to effectively use OSPF. The benefits of having a single area include simplicity, ease of troubleshooting, and so on.
A contiguous backbone area must be present, and all areas must have a connection to the backbone.
Explicit topology (shortest path) has precedence over any IP addressing schemes ...