A network engineer connects routers R5 and R6 to the same Ethernet LAN and configures them to use OSPFv2. Which answers describe a combination of settings that would prevent the two routers from becoming OSPF neighbors?

(A) Both routers’ interface IP addresses are in the same subnet
(B) Both routers’ OSPF process uses process ID 3
(C) Both routers’ OSPF process uses router ID 42.42.42.42
(D) Both routers’ interfaces use an OSPF Dead interval of 80

Both routers’ OSPF process uses router ID 42.42.42.42 is the correct answer. The use of an identical OSPF router ID (RID) on the two routers prevents them from becoming neighbors. Both routers must have the same Dead interval, so both using a Dead interval of 80 causes no issues. The two routers can use any OSPF process ID either the same or a different value, making that answer incorrect. Finally, the two routers’ IP addresses must be in the same subnet, so again that scenario does not prevent R5 and R6 from becoming neighbors.