Which of the following must match for forming OSPF adjacency?

For forming OSPF adjacency, it is critical that the MTU (Maximum Transmission Unit) size must match on both routers participating in the OSPF network. This requirement ensures that packets can be transmitted without fragmentation, maintaining the integrity and performance of the OSPF routing process. If the MTU sizes do not match, the routers will not be able to successfully form adjacencies, which are essential for OSPF to function correctly.

In OSPF, router ID, area types, and various operational commands do influence many aspects of OSPF operation, but they do not directly prevent adjacency formation as the MTU size does. Mismatched areas may affect routing updates, and differing router IDs may lead to various operational challenges, but the immediate requirement to establish adjacency is strictly the MTU size match. This focus on MTU underlines the importance of ensuring that the physical characteristics of the network path are compatible to support OSPF operations effectively.