What must be consistent among OSPF routers to ensure they form an adjacency?

To ensure that OSPF routers form an adjacency, the area configuration must be consistent among them. OSPF uses a hierarchical structure based on areas, and for routers to successfully establish a neighbor relationship and share routing information, they need to be configured for the same OSPF area.

When two routers are in the same area, they can exchange routing updates and maintain the OSPF database in a synchronized manner. If the area configurations differ, the routers will not be able to establish an adjacency, as they will consider themselves to be in different logical networks. Therefore, consistency in area configuration is paramount for OSPF routers to communicate effectively and form stable adjacencies.

Router ID, OSPF versions, and interface types, while important for overall configuration and operation, do not directly influence the ability of routers to form adjacencies in the same way that consistent area configuration does.