What is DBD?

A DBD (Database Description packet) is the OSPF message that lists the LSAs a router holds, so a new neighbour can work out what it is missing.

OSPF has five packet types: Hello, Database Description, Link State Request, Link State Update and Link State Acknowledgement. DBDs are type 2, and they are used in two neighbour states. In ExStart, the routers send empty DBDs to agree who is master (the higher router ID) and to set a starting sequence number. In Exchange, they send DBDs that carry LSA headers only: type, link state ID, advertising router, sequence number, age and checksum, but not the LSA bodies. Flags in each packet mark the first packet (I), whether more follow (M) and master or slave (MS), and the master drives the exchange by sequence number. Comparing the headers against its own LSDB tells each router which LSAs it lacks or holds an older copy of. It then asks for those with Link State Requests and receives them in Link State Updates during the Loading state, before the adjacency reaches Full.

DBDs also carry the sending interface's MTU, and a router rejects DBDs from a neighbour whose MTU is larger than its own. That is why an MTU mismatch is the classic cause of a neighbour stuck in ExStart or Exchange: Hellos get through, so the neighbour appears, but the database exchange never completes. Fix it by matching the MTU on both ends rather than by turning off the check. The OSPF Fundamentals lesson covers each adjacency state, and the follow-up packets are described under LSR and LSU.