What are the five OSPF packet types and what does each do?
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Type 1 Hello finds and keeps neighbors. Type 2 Database Description (DBD) lists LSA headers during the database exchange. Type 3 Link State Request (LSR) asks for LSAs the router is missing. Type 4 Link State Update (LSU) carries the LSAs. Type 5 Link State Acknowledgment (LSAck) confirms receipt.
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Where each appears
| Type | Name | Used in |
|---|---|---|
| 1 | Hello | Neighbor discovery, keepalive, DR election |
| 2 | DBD | EXSTART (master and slave) and EXCHANGE (LSA headers) |
| 3 | LSR | LOADING, and EXCHANGE as soon as gaps are seen |
| 4 | LSU | Answers to LSRs and all later flooding |
| 5 | LSAck | Acknowledges LSUs so flooding is reliable |
All five ride directly on IP, protocol 89. OSPF builds its own reliability: an LSU that is not
acknowledged is retransmitted every retransmit-interval (5 seconds by default).
The common header
Version (2 for OSPFv2, 3 for OSPFv3), type, length, router ID, area ID, checksum, and authentication type and data in OSPFv2.
They will ask next
Is an LSA the same thing as an LSU?
No. The LSU is the packet. One LSU can carry many LSAs, and each LSA has its own header with type, link state ID, advertising router, sequence number and age.
Traps
- Confusing packet types (1 to 5) with LSA types (1 to 7). Interviewers mix them on purpose.