Refer to the exhibit. A company recently deployed a new network using OSPF in the core to share routes. The network administrator selected OSPF as the routing protocol because of its ability to…

Mismatched OSPF hello or dead timers would also prevent OSPF adjacency from forming, causing the neighbors to get stuck in the Init or 2-Way state. However, MTU mismatch (D) is a more fundamental issue that can prevent the successful exchange of even small OSPF packets, leading to the "link failed to come up" description.

OSPF router IDs must be unique within an OSPF domain, but they do not need to be in different subnets. Duplicate router IDs would cause issues, but not typically prevent the link from coming up or initial OSPF hellos.

OSPF process IDs are locally significant to a router and do not need to match between neighboring routers for adjacency to form. They are used to distinguish multiple OSPF instances on a single device.

Mismatched MTU (Maximum Transmission Unit) settings on directly connected interfaces is a common reason for OSPF adjacency failure. OSPF performs an MTU check during the ExStart/Exchange state of neighbor establishment; if the MTU values do not match, the routers will get stuck in these states and will not form a full adjacency, effectively causing the "link" for OSPF communication to fail. Ensuring matching MTUs on both R1 and R2 interfaces will allow OSPF adjacency to form correctly.