When troubleshooting OSPF, what must you check if routers are not forming an adjacency?

When troubleshooting OSPF adjacency issues, all mentioned aspects are crucial components to examine, making the comprehensive choice the most relevant.

Router priority settings play an important role in determining which router becomes the designated router (DR) and which becomes the backup designated router (BDR) during the neighbor formation process. Incorrect priority settings can hinder the adjacency formation, especially in broadcast and non-broadcast multi-access (NBMA) networks, where decisions on DR and BDR are made based on these priorities.

Link configuration must also be verified. This includes checking for correct IP address assignments, subnet masks, and the physical or logical state of the interfaces. If the interfaces involved in OSPF are down or misconfigured, routers may fail to establish an adjacency.

The OSPF process ID is necessary for routers to recognize and correctly participate in the same OSPF domain. If the process ID does not match on either side of a link where OSPF is to run, adjacency will not form.

Considering all these factors together ensures a thorough approach to diagnosing and resolving OSPF adjacency issues, emphasizing the interconnectedness of these elements in the OSPF protocol.