What are two reasons that will prevent routers from forming an OSPFv2 adjacency? (Choose two.)

Question:
What are two reasons that will prevent routers from forming an OSPFv2 adjacency? (Choose two.)

• mismatched subnet masks on the link interfaces
• use of private IP addresses on the link interfaces
• one router connecting to a FastEthernet port on the switch and the other connecting to a GigabitEthernet port
• a mismatched Cisco IOS version that is used
• mismatched OSPF Hello or Dead timers

Explanation: C and D. There may be several reasons why routers running OSPF will fail to form an OSPF adjacency, including subnet masks not matching, OSPF hello or dead timers not matching, OSPF network types not matching, or a missing or incorrect OSPF network command. Mismatched IOS versions, the use of private IP addresses, and different types of interface ports do not cause an OSPF adjacency to fail to form between two routers.

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