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OSPF packet formats
OSPF packets are directly encapsulated into IP packets. OSPF uses the IP protocol number 89. The format
of an OSPF LSU packet is shown in Figure 24.
Figure 24 OSPF packet
format
OSPF packet header
OSPF packets are classified into five types that have the same packet header.
Figure 25 OSPF packet header
Major fields of the OSPF packet header are as follows:
• Version—OSPF version number, which is 2 for OSPFv2.
• Type—OSPF packet type from 1 to 5, corresponding to 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, ranging from 0 to 2, corresponding to 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.
NOTE:
MD5 authentication data is added followin
an OSPF packet rather than contained in the Authentication
field.
Hello packet
A router sends hello packets periodically to find and maintain neighbor relationships, and to elect the DR
or BDR, including information about values of timers, DR, BDR, and neighbors that are already known.
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