OSPF Packet Types

OSPF uses five packet types:

• Hello packet

  OSPF uses Hello packets to implement the following functions:

  • Neighbor discovery: OSPF routers periodically send Hello packets on every OSPF-enabled interface, and negotiate the specific parameters in the Hello packets with other routers that receive Hello packets to determine whether to establish an OSPF neighbor relationship.
  • Bidirectional communications: If a router sees its own router ID in the neighbor list of a received Hello packet, bidirectional communication has been established between this receiving router and the router that originates the packet. An OSPF neighbor relationship has been established between the two routers.
  • Designated router (DR) and backup designated router (BDR) election: Hello packets contain information about the DR priority and router ID. Each router writes its selected DR and BDR to the DR and BDR fields of Hello packets for DR and BDR election. For details, see DR/BDR Election.
  • Keepalives: After an OSPF neighbor relationship is established, OSPF-enabled interfaces still periodically send Hello packets to maintain the neighbor relationship. If an interface does not receive a Hello packet from a neighbor within a certain period, it terminates the neighbor relationship with this neighbor.

• Database Description (DD) packet

  During adjacency initialization, two routers send DD packets to negotiate their master/slave relationship. In this phase, the DD packets do not contain an LSA header. When two routers exchange DD packets, one functions as the master and the other functions as the slave. The master defines a start sequence number and increments the sequence number by 1 each time it sends a DD packet. After the slave receives a DD packet, it uses the sequence number carried in the DD packet for acknowledgement. After an adjacency is established, routers use DD packets to describe their own LSDBs for LSDB synchronization. A DD packet contains the header of each LSA in an LSDB and is the summary of all LSAs. An LSA header uniquely identifies an LSA. The LSA header occupies only a small portion of the LSA, which reduces the amount of traffic transmitted between routers. A neighbor can use the LSA header to check whether it already has the LSA.

• Link State Request (LSR) packet

  After two routers have exchanged DD packets, they send LSR packets to request each other's LSAs. The LSR packets contain the summaries of the requested LSAs.

• Link State Update (LSU) packet

  A router uses an LSU packet to transmit LSAs requested by its neighbors or to flood its own updated LSAs. The LSU packet contains a set of LSAs. To ensure reliable LSA flooding, the router uses an LSAck packet to acknowledge the LSAs contained in an LSU packet that it received from a neighbor. If an LSA fails to be acknowledged, the router directly retransmits the LSA to the neighbor.

• Link State Acknowledgement (LSAck) packet

  A router uses an LSAck packet to acknowledge the LSAs contained in a received LSU packet. The LSAs can be acknowledged using LSA headers.