Link-state packets are the tools used by OSPF to help determine the fastest available route for a packet. OSPF uses the following link-state packets (LSPs) to establish and maintain neighbor adjacencies and exchange routing updates. Each packet serves a specific purpose in the OSPF routing process, as follows:
- Type 1: Hello packet – This is used to establish and maintain adjacency with other OSPF routers.
- Type 2: Database Description (DBD) packet – This contains an abbreviated list of the LSDB of the sending router 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 – This is used to reply to LSRs and to announce new information. LSUs contain several 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.
The table summarizes the five different types of LSPs used by OSPFv2. OSPFv3 has similar packet types.
| Type | Packet Name | Description |
|---|---|---|
| 1 | Hello | Discovers neighbors and builds adjacencies between them |
| 2 | Database Description (DBD) | Checks for database synchronization between routers |
| 3 | Link-State Request (LSR) | Requests specific link-state records from router to router |
| 4 | Link-State Update (LSU) | Sends specifically requested link-state records |
| 5 | Link-State Acknowledgment (LSAck) | Acknowledges the other packet types |
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