Configuring IS-IS on Broadcast Multiaccess Links

Even though IS-IS operation on broadcast links differs a little from IS-IS operation on point-to-point links, the basic configuration is not significantly different. With broadcast links, you also activate the IS-IS routing process and then enable IS-IS routing on the interfaces where adjacencies are to be formed . In the LAN configuration example shown Figure 9-3, three routers (RT1, RT2, and RT3) are interconnected over an Ethernet link. All routers are in the same area.

Figure 9-3. Ethernet LAN configuration example.

A significant difference between a point-to-point scenario and this example is that, in this case, one of the routers is elected to be the designated intermediate system (DIS) to simplify database synchronization over the broadcast medium. The DIS effectively emulates the LAN as a pseudonode and uses an ID based on its system ID to represent the pseudonode. The pseudonode ID is also used by all routers connected to the LAN (including the DIS) as their circuit ID. The circuit ID associates an IS-IS interface with a link, and it is shown in the show clns interface output for the respective interface.