There are three fundamental requirements for OSPF configuration, and they are illustrated below.
Requirement One: When implementing inter-area OSPF, Area 0 must exist as the backbone. In the example illustrated below, Area 7 is incorrectly configured. It should be Area 0.
Requirement Two: All non-backbone areas must have an ABR with an interface in the backbone Area 0. In the example illustrated below, Area 4 does not have an ABR that interfaces with Area 0, which is incorrect.
Requirement Three: Area 0 must be contiguous. Area 0 cannot exist in two places at once, as illustrated in the drawing below.
Stuck-in-Active: Journal of an IT-Network Administrator 