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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)
                     9  Open Shortest Path First (OSPF)

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OSPF General Operation and Message Types
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Other Interior Routing Protocols
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OSPF Message Formats
(Page 1 of 7)

OSPF uses five different types of messages to communicate both link-state and general information between routers within an autonomous system or area. To help illustrate better how the OSPF messages are used, it's worth taking a quick look at the format used for each of these messages. Well, I guess I am being somewhat liberal in my use of the term “quick” here. J

OSPF Common Header Format

Naturally, each type of OSPF message includes a slightly different set of information—otherwise, they wouldn't be different message types! However, they all share a similar message structure, beginning with a shared 24-byte header. This common header allows certain standard information to be conveyed in a consistent manner, such as the number of the version of OSPF that generated the message. It also allows a device receiving an OSPF message to quickly determine which type of message it has received, so it knows whether or not it needs to bother examining the rest of the message.Table 126 and Figure 184 show the common OSPF header format.


Table 126: OSPF Common Header Format

Field Name

Size (bytes)

Description

Version #

1

Version Number: Set to 2 for OSPF version 2.

Type

1

 

Packet Length

2

Packet Length: The length of the message, in bytes, including the 24 bytes of this header.

Router ID

4

Router ID: The ID of the router that generated this message (generally its IP address on the interface over which the message was sent).

Area ID

4

Area ID: An identification of the OSPF area to which this message belongs, when areas are used.

Checksum

2

Checksum: A 16-bit checksum computed in a manner similar to a standard IP checksum. The entire message is included in the calculation except the Authentication field.

AuType

2

 

Authentication

8

Authentication: A 64-bit field used for authentication of the message, as needed.



Figure 184: OSPF Common Header Format

 


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