12.4.2. Network links

A network links advertisement is generated for every transit multi-access network. (A transit network is a network having two or more attached routers). The network links advertisement describes all the routers that are attached to the network.

The Designated Router for the network originates the advertisement. The Designated Router originates the advertisement only if it is fully adjacent to at least one other router on the network. The network links advertisement is flooded throughout the area that contains the transit network, and no further. The networks links advertisement lists those routers that are fully adjacent to the Designated Router; each fully adjacent router is identified by its OSPF Router ID. The Designated Router includes itself in this list.

The Link State ID for a network links advertisement is the IP interface address of the Designated Router. This value, masked by the network's address mask (which is also contained in the network links advertisement) yields the network's IP address.

A router that has formerly been the Designated Router for a network, but is no longer, should flush the network links advertisement that it had previously originated. This advertisement is no longer used in the routing table calculation. It is flushed by prematurely incrementing the advertisement's age to MaxAge and reflooding (see Section 14.1). In addition, in those rare cases where a router's Router ID has changed, any network links advertisements that were originated with the router's previous Router ID must be flushed. Since the router may have no idea what it's previous Router ID might have been, these network links advertisements are indicated by having their Link State ID equal to one of the router's IP interface addresses and their Advertising Router not equal to the router's current Router ID (see Section 13.4 for more details).

As an example of a network links advertisement, again consider the area configuration in Figure 6. Network links advertisements are originated for Network N3 in Area 1, Networks N6 and N8 in Area 2, and Network N9 in Area 3. Assuming that Router RT4 has been selected as the Designated Router for Network N3, the following network links advertisement is generated by RT4 on behalf of Network N3 (see Figure 15 for the address assignments):

              ; network links advertisement for Network N3

              LS age = 0                     ;always true on origination
              Options = (T-bit|E-bit)        ;TOS-capable
              LS type = 2                    ;indicates network links
              Link State ID = 192.1.1.4      ;IP address of Desig. Rtr.
              Advertising Router = 192.1.1.4 ;RT4's Router ID
              Network Mask = 0xffffff00
                     Attached Router = 192.1.1.4    ;Router ID
                     Attached Router = 192.1.1.1    ;Router ID
                     Attached Router = 192.1.1.2    ;Router ID
                     Attached Router = 192.1.1.3    ;Router ID