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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 informationotherwise, 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
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Size (bytes)
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Description
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Version
#
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1
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Version Number:
Set to 2 for OSPF version 2.
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Type
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1
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Packet
Length
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2
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Packet Length:
The length of the message, in bytes, including the 24 bytes of this
header.
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Router
ID
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4
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Router
ID: The ID of the router that generated this message (generally
its IP address on the interface over which the message was sent).
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Area ID
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4
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Area ID: An identification
of the OSPF area to which this message belongs, when areas are used.
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Checksum
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2
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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.
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AuType
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2
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Authentication
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8
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Authentication:
A 64-bit field used for authentication of the message, as needed.
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