There are five types of routing protocol packets used in OSPF. They are listed below with comments about their significance. Each packet type is encapsulated directly in the IP header with protocol number 89.
Do not confuse this with LSA types.
TYPE 1 (Hello) – Used to discover neighbors with common criteria for OSPF adjacency, such as Area ID. The default hello interval is 10 seconds. Once neighbors have agreed upon the parameters of the Type 1 packet, they progress to sending Type 2 packets.
TYPE 2 (DBD) – This is a Database Descriptor packet. It is a summary of the OSPF database that each neighbor shares in order to identify routes that they are missing from their OSPF databases. This is essentially used to make sure databases are in sync.
TYPE 3 (LSR) – This is a Link State Request packet. It is sent to request specific LSAs from another router when database synchronization in Type 2 determines there are missing routes. Acknowledgement is required by a Type 5 packet.
TYPE 4 (LSU) – The Link State Update is response to the Type 3 packet, carrying specific information requested by another router. Can be sent either by flooding updates or providing specific response to other routers. (If flooding occurs, neighbors are responsible for re-encapsulation of new LSUs.) Acknowledgement is required by a Type 5 packet.
TYPE 5 (LSAck) – The Link State Acknowledgement is a confirmation of receipt of Type 3 and Type 4 packets. Multiple LSAs can be acknowledged by a single LSAck packet.
Stuck-in-Active: Journal of an IT-Network Administrator 