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).