All OSPF packets begin with a 24-byte header, as illustrated in Figure 46-2.
Figure 46-2 OSPF Packets Consist of Nine Fields
The following descriptions summarize the header fields illustrated in Figure 46-2.
•Version number—Identifies the OSPF version used.
•Type—Identifies the OSPF packet type as one of the following:
–Hello—Establishes and maintains neighbor relationships.
–Database description—Describes the contents of the topological database. These messages are exchanged when an adjacency is initialized.
–Link-state request—Requests pieces of the topological database from neighbor routers. These messages are exchanged after a router discovers (by examining database-description packets) that parts of its topological database are outdated.
–Link-state update—Responds to a link-state request packet. These messages also are used for the regular dispersal of LSAs. Several LSAs can be included within a single link-state update packet.
–Link-state acknowledgment—Acknowledges link-state update packets.
•Packet length—Specifies the packet length, including the OSPF header, in bytes.
•Router ID—Identifies the source of the packet.
•Area ID—Identifies the area to which the packet belongs. All OSPF packets are associated with a single area.
•Checksum—Checks the entire packet contents for any damage suffered in transit.
•Authentication type—Contains the authentication type. All OSPF protocol exchanges are authenticated. The authentication type is configurable on per-area basis.
•Authentication—Contains authentication information.
•Data—Contains encapsulated upper-layer information.