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OSPF neighbors on PRI, BRI or dialer interfaces are stuck in exstart state
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.
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.