Core Issue
When Open Shortest Path First (OSPF) is enabled on a router, the status of the OSPF relationship between the OSPF neighbors transitions through several states. If the neighbors are able to form adjacencies successfully, this changes to a full state.
However, the OSPF state between neighbors connected to PRI, BRI and dialer interfaces may not transition to a full state and get stuck in exstart state instead. This occurs because of a Database Descriptor (DBD) synchronization failure when multiple neighbors are simultaneously connected through the same interface.
The default OSPF network type on PRI, BRI and dialer interfaces is point-to-point and multicasts are used for DBD synchronization. Therefore, DBD packets meant for a single neighbor are heard by all the neighbors on the segment. This results in a mismatch in the sequence number and the master and slave relation.
Resolution
To make OSPF routers on PRI, BRI or dialer interfaces establish adjacencies successfully, configure the network type as point-to-multipoint. This makes the routers use unicast packets for DBD synchronization.
To configure the OSPF network type as point-to-multipoint, issue the ip ospf network command with the point-to-multipoint keyword in interface configuration mode.
For more information, refer to Why Doesn't OSPF Form Adjacency on a PRI, BRI, or Dialer Interface?.
show ip ospf neighbors
Exstart or exchange
