OSPF uses link-state packets (LSPs) to establish and maintain neighbor adjacencies and exchange routing updates.
The figure shows the five different types of LSPs used by OSPF. Each packet serves a specific purpose in the OSPF routing process:
- Type 1: Hello packet - Used to establish and maintain adjacency with other OSPF routers.
- Type 2: Database Description (DBD) packet - Contains an abbreviated list of the sending router’s LSDB and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all link-state routers within an area to construct an accurate SPF tree.
- Type 3: Link-State Request (LSR) packet - Receiving routers can then request more information about any entry in the DBD by sending an LSR.
- Type 4: Link-State Update (LSU) packet - Used to reply to LSRs and to announce new information. LSUs contain seven different types of LSAs.
- Type 5: Link-State Acknowledgment (LSAck) packet - When an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.