All OSPF packets start with a 24-octet header that contains information about the OSPF version, the packet type and length, the ID of the router that transmits the packet, and the ID of the OSPF area that sends the packet. An OSPF packet is one of the following types:
The router transmitted hello packets between neighbors and never forwards them. The Hello protocol requires routers to send hello packets to neighbors at predefined hello intervals. A neighbor router that does not receive a hello packet declares the other router dead.
The router exchanges DD packets after neighboring routers establish a link, which synchronizes their LSDBs.
Link-state request packets describe one or more link-state advertisements that a router requests from its neighbor. Routers send link-state requests if the information received in DD packets from a neighbor is not consistent with its own link-state database.
Link-state update packets contain one or more LSAs and the router sends them following a change in network conditions.
The router sends link-state acknowledgement packets to acknowledge receipt of link-state updates. Link-state acknowledgement packets contain the headers of the received LSAs.