12.1.4. Link State ID

This field identifies the piece of the routing domain that is being described by the advertisement. Depending on the advertisement's LS type, the Link State ID takes on the values listed in Table 16.

Actually, for Type 3 summary link (LS type = 3) advertisements and AS external link (LS type = 5) advertisements, the Link State ID may additionally have one or more of the destination network's "host" bits set. For example, when originating an AS external link for the network 10.0.0.0 with mask of 255.0.0.0, the Link State ID can be set to anything in the range 10.0.0.0 through 10.255.255.255 inclusive (although 10.0.0.0 should be used whenever possible). The freedom to set certain host bits allows a router to originate separate advertisements for two networks having the same address but different masks. See Appendix F for details.

           LS Type   Advertisement description
           __________________________________________________
           1         These are the router links
                     advertisements. They describe the
                     collected states of the router's
                     interfaces. For more information,
                     consult Section 12.4.1.
           __________________________________________________
           2         These are the network links
                     advertisements. They describe the set
                     of routers attached to the network. For
                     more information, consult
                     Section 12.4.2.
           __________________________________________________
           3 or 4    These are the summary link
                     advertisements. They describe
                     inter-area routes, and enable the
                     condensation of routing information at
                     area borders. Originated by area border
                     routers, the Type 3 advertisements
                     describe routes to networks while the
                     Type 4 advertisements describe routes to
                     AS boundary routers.
           __________________________________________________
           5         These are the AS external link
                     advertisements. Originated by AS
                     boundary routers, they describe routes
                     to destinations external to the
                     Autonomous System. A default route for
                     the Autonomous System can also be
                     described by an AS external link
                     advertisement.

               Table 15: OSPF link state advertisements.

            LS Type   Link State ID
            _______________________________________________
            1         The originating router's Router ID.
            2         The IP interface address of the
                      network's Designated Router.
            3         The destination network's IP address.
            4         The Router ID of the described AS
                      boundary router.
            5         The destination network's IP address.

              Table 16: The advertisement's Link State ID.

When the link state advertisement is describing a network (LS type = 2, 3 or 5), the network's IP address is easily derived by masking the Link State ID with the network/subnet mask contained in the body of the link state advertisement. When the link state advertisement is describing a router (LS type = 1 or 4), the Link State ID is always the described router's OSPF Router ID.

When an AS external advertisement (LS Type = 5) is describing a default route, its Link State ID is set to DefaultDestination (0.0.0.0).