Introduction to OSPF 283
Figure 80 OSPF packet format
OSPF packet header
OSPF packets are classified into five types that have the same packet header, as
shown below.
Figure 81 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.
n
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:
IP header OSPF packet header Number of LSAs LSA header LSA Data
Version Type
Router ID
Area ID
Checksum AuType
Packet length
Authentication
Authentication
0 7 15 31
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