- a mismatched Cisco IOS version that is used
- mismatched OSPF Hello or Dead timers
- 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
Explanation: There may be several reasons why two routers running OSPF will fail to form an OSPF adjacency, including these:
The subnet masks do not match, causing the routers to be on separate networks.
OSPF Hello or Dead Timers do not match.
OSPF network types do not match.
There is a missing or incorrect OSPF network command.
Mismatched IOS versions, the use of private IP addresses, and different types of interface ports used on a switch are not causes for an OSPF adjacency failing to form between two routers.