OSPF Packet Types Explained: The 5 Key Messages in OSPF
Open Shortest Path First (OSPF) uses five distinct packet types to perform its functions as a link-state routing protocol. These packet types allow OSPF routers to discover neighbors, exchange routing information, ensure synchronization of link-state databases (LSDBs), and maintain reliability in the network. Each packet type plays a crucial role during different stages of neighbor formation and database synchronization.
In this article, we'll break down each of these OSPF packet types, explaining their purpose and behavior within the OSPF finite state machine.
1. Hello Packet
Purpose: Discover and maintain neighbor relationships.
Sent periodically on all OSPF-enabled interfaces.
Used to discover new neighbors and ensure existing neighbors are still active.
Contains key parameters such as Router ID, Hello/Dead intervals, network mask, options, and designated router info.
Neighbor relationships only form between routers with matching parameters.
Used in states: Down, Init, 2-Way
2. Database Description (DBD or DDP) Packet
Purpose: Summarize LSDB contents to neighbor routers.
Exchanged during the Exchange state.
Routers describe their link-state database content using LSA headers.
Helps routers identify which LSAs are missing or outdated.
Includes sequence numbers and master/slave negotiation in ExStart.
Used in states: ExStart, Exchange
3. Link-State Request (LSR) Packet
Purpose: Request missing or outdated LSAs from a neighbor.
Sent by a router when it detects a missing or stale LSA.
Generated after comparing LSA summaries received in DBD packets.
Requests are specific and precise (type, ID, advertising router).
Used in state: Loading
4. Link-State Update (LSU) Packet
Purpose: Transmit full LSA contents in response to LSRs.
Contains one or more complete LSAs.
Used to update a neighbor’s database with detailed link-state information.
Sent in direct response to LSRs, but can also be used to flood LSAs.
Used in state: Loading
5. Link-State Acknowledgment (LSAck) Packet
Purpose: Ensure reliable LSA flooding and delivery.
Acknowledges receipt of LSUs.
Prevents retransmission and confirms LSA reception.
Can be sent as direct, delayed, or summary acknowledgments.
Used in state: Loading
Summary Table
| Type | Packet Name | Functional Overview |
|---|---|---|
| 1 | Hello | Discover & maintain neighbor relationships |
| 2 | Database Description (DBD) | Summarize LSDB contents |
| 3 | Link-State Request (LSR) | Request missing LSAs from neighbor |
| 4 | Link-State Update (LSU) | Send full LSAs to update neighbor's database |
| 5 | Link-State Acknowledgment | Confirm reliable LSA delivery and flooding |
Post a Comment