OSPF Packet Format

Packet Format: Taken from,

http://docwiki.cisco.com/wiki/Open_Shortest_Path_First 

This is useful when you are looking at a packet capture of an OSPF packet. Same can be said for any packet format visualization of any protocol. I am reviewing some OSPF reading material and figured I would post this.

All OSPF packets begin with a 24-byte header, as illustrated in Figure: OSPF Packets Consist of Nine Fields.

Figure: OSPF Packets Consist of Nine Fields

The following descriptions summarize the header fields:

• 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after a router discovers (by examining database-description packets) that parts of its topological database are outdated.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.