Common causes of this problems are:

- The OSPF network is a non-broadcast multiple access (NBMA) network, and no neighbor is configured.

- The interval at which the OSPF neighbor sends Hello packets is longer than the aging time of the neighbor relationship.
- In a stub area or not-so-stubby area (NSSA) area, some devices are not configured as stub routers or NSSA routers.
- The configuration of OSPF verification is incorrect.
- The OSPF router IDs on two ends of the OSPF link are different.
- The network types on two ends of the OSPF link are different.
- The MTU values on two ends of the OSPF link are different.
- OSPF is not enabled on the local or remote device.
- The area numbers on two ends of the OSPF link are different.