OSPF Packet Format
OSPF is a routing protocol designed specifically for the TCP/IP environment, and it runs directly on top of IP protocol 89. Another noteworthy point about OSPF is that it does not perform fragmentation and reassembly; these functions are performed at the IP layer.
Every OSPF packet shares a common 24-byte protocol header, as shown in Figure 9-1. Routing protocol packets are sent with a type of service of zero.
Figure 9-1. The 24-byte Common OSPF Header Shared by All Protocol Packets
Figure 9-1. The 24-byte Common OSPF Header Shared by All Protocol Packets
OSPF has five types of routing protocol packets; the Type field in the protocol header identifies each one as follows:
Version
This is one byte; the current version of OSPF is two. Type
This is used to identify the OSPF packet. Five types of OSPF packets exist:
|
Type |
Description |
|
1 |
Hello |
|
2 |
Database description packet |
|
3 |
Link-state request |
|
4 |
Link-state update |
|
5 |
Link-state acknowledgment |
Each of the packet types is discussed in detail later in this chapter.
This four-byte field is used to identify the router originating the OSPF packet.
This four-byte field is used to identify the area to which the packet belongs. All OSPF packets are sent one hop, except on virtual links. In virtual links, which are used to patch the backbone area, this field is set to 0.0.0.0.
• Checksum
OSPF uses regular IP checksum, which covers the complete packet except for the eight bytes of authentication.
• Authentication
This eight-byte field is used for authentication of the process used.
Continue reading here: OSPF Packet Types
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