For OSPF neighbors to reach Full state, they must exchange LSAs (Link State Advertisements) properly and synchronize their databases.
Possible Causes Preventing OSPF from Entering Full State:
# A. A link works abnormally
* If the physical or Layer 2 link is unstable, OSPF Hello packets may be lost, preventing the Full state.
# B. The OSPF network types on both ends of the link are inconsistent
* OSPF network types (Broadcast, P2P, NBMA, etc.) must match on both ends.
* Example: If one router is P2P and the other is Broadcast, adjacency issues occur.
# C. The router IDs of neighbors are the same
* Router ID conflicts prevent OSPF adjacencies from forming.
* Each router must have a unique Router ID.
# D. The OSPF MTU values of interfaces on both ends of the link are different
* If MTU settings do not match, OSPF Database Description (DD) packets may be dropped, preventing Full state.
Reference from Huawei HCIE-Datacom Documentation:
* Huawei OSPF Configuration Guide - Troubleshooting OSPF Neighbor Issues
* HCIE-Datacom Study Material - OSPF Adjacency and Route Synchronization