A.4.1 The Link State Advertisement header

All link state advertisements begin with a common 20 byte header. This header contains enough information to uniquely identify the advertisement (LS type, Link State ID, and Advertising Router). Multiple instances of the link state advertisement may exist in the routing domain at the same time. It is then necessary to determine which instance is more recent. This is accomplished by examining the LS age, LS sequence number and LS checksum fields that are also contained in the link state advertisement header.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |    Options    |    LS type    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age

The time in seconds since the link state advertisement was originated.

Options

The optional capabilities supported by the described portion of the routing domain. OSPF's optional capabilities are documented in Section A.2.

LS type

The type of the link state advertisement. Each link state type has a separate advertisement format. The link state types are as follows (see Section 12.1.3 for further explanation):

                        LS Type   Description
                        ___________________________________
                        1         Router links
                        2         Network links
                        3         Summary link (IP network)
                        4         Summary link (ASBR)
                        5         AS external link

Link State ID

This field identifies the portion of the internet environment that is being described by the advertisement. The contents of this field depend on the advertisement's LS type. For example, in network links advertisements the Link State ID is set to the IP interface address of the network's Designated Router (from which the network's IP address can be derived). The Link State ID is further discussed in Section 12.1.4.

Advertising Router

The Router ID of the router that originated the link state advertisement. For example, in network links advertisements this field is set to the Router ID of the network's Designated Router.

LS sequence number

Detects old or duplicate link state advertisements. Successive instances of a link state advertisement are given successive LS sequence numbers. See Section 12.1.6 for more details.

LS checksum

The Fletcher checksum of the complete contents of the link state advertisement, including the link state advertisement header but excepting the LS age field. See Section 12.1.7 for more details.

length

The length in bytes of the link state advertisement. This includes the 20 byte link state advertisement header.