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OSPF Message Formats
(Page 7 of 7)
Link State Advertisements (LSAs) and the LSA Header Format
As we saw above, several of the message
types include link state advertisements (LSAs), which are the
fields that actually carry topological information about the LSDB. There
are several types of LSAs, which are used to convey information about
different types of links. Like the OSPF messages themselves, each LSA
has a common header with 20 bytes, and then a number of additional fields
that describe the link.
LSA Header
The LSA header contains sufficient
information to identify the link. It uses the subfield structure in
Table 132
and Figure 190.
Table 132: OSPF Link State Advertisement Header Format
Subfield
Name
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Size (bytes)
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Description
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LS Age
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2
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LS Age: The number
of seconds elapsed since the LSA was created.
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Options
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1
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Options:
Indicates which of several optional OSPF capabilities the router supports.
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LS Type
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1
|
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Link
State ID
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4
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Link
State ID: Identifies the link. This usually is the IP address
of either the router or the network the link represents.
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Advertising
Router
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4
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Advertising Router:
The ID of the router originating the LSA.
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LS
Sequence Number
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4
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LS Sequence
Number: A sequence number used to detect old or duplicate LSAs.
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LS Checksum
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2
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LS Checksum: A
checksum of the LSA, for data corruption protection.
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Length
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2
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Length:
The length of the LSA, including the 20 bytes of the header.
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Figure 190: OSPF Link State Advertisement Header Format
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LSA Body
Following the LSA header comes the
body of the LSA, the specific fields of which depend on the value of
the LS Type field. It would take several more pages
to describe all these subfields, and I already probably went overboard
in describing each message type, so I will refer you to Appendix A of
RFC 2328 if you want all the details. By way of quick summary:
- For normal links to a router, the LSA includes
an identification of the router and the metric to reach it, as well
as details about the router such as whether it is a boundary or area
border router.
- LSAs for networks include a subnet mask and information
about other routers on the network.
- Summary LSAs include a metric and a summarized
address, as well as a subnet mask.
- External LSAs include a number of additional
fields to allow the external router to be communicated.
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