HP 4800G Series Configuration Manual page 445

Figure 1-8 OSPF packet format
OSPF packet header
OSPF packets are classified into five types that have the same packet header, as shown below.
Figure 1-9 OSPF packet header
Version: OSPF version number, which is 2 for OSPFv2.
Type: OSPF packet type from 1 to 5, corresponding with hello, DD, LSR, LSU and LSAck
respectively.
Packet length: Total length of the OSPF packet in bytes, including the header.
Router ID: ID of the advertising router.
Area ID: ID of the area where the advertising router resides.
Checksum: Checksum of the message.
Autype: Authentication type from 0 to 2, corresponding with non-authentication, simple (plaintext)
authentication and MD5 authentication respectively.
Authentication: Information determined by authentication type. It is not defined for authentication
type 0. It is defined as password information for authentication type 1, and defined as Key ID, MD5
authentication data length and sequence number for authentication type 2.
MD5 authentication data is added following an OSPF packet rather than contained in the Authentication
field.
Hello packet
A router sends hello packets periodically to neighbors to find and maintain neighbor relationships and to
elect the DR/BDR, including information about values of timers, DR, BDR and neighbors already known.
The format is shown below:
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