OSPF is a routing protocol designed specifically for
the TCP/IP environment. 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 the graphic. Routing
protocol packets are sent with a type of service of zero. There are a variety of
OSPF packet types. However, each type shares a common OSPF header. The structure
of these 24-byte OSPF headers is as follows:
- Version --- This portion of the header is one byte long and indicates what
version of OSPF is running (Note: the current version of OSPF is Version 2).
- Type --- The second byte of the header identifies the type of OSPF packet.
There are five OSPF packet types, as indicated in Figure
.
- Packet Length --- A two byte field that indicates the entire length of the
packet.
- Router ID --- A four byte field used to identify the router originating the
OSPF packet.
- Area ID --- A four byte field used to identify the area to which the packet
belongs.
- Checksum --- A two byte checksum field used to check the message for
errors. Good packets are retained and damaged packets are discarded.
- Authentication Type --- A two byte field that identifies the form of
identification that is used on this packet.
- Authentication --- An eight byte field used for authentication data that
allows the receiver to authenticate the sender.
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