OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

4.4. Designated Routers

Broadcast network media such as Ethernet presents interesting problems for link state protocols, both in terms of database synchronization and SPF calculations. Think of six routers sharing a broadcast link, as in Figure 4.22. If each of the six routers forms an adjacency with each of its neighbors in the link, there will be a total of 15 adjacencies, as shown in the illustration. In fact the number of adjacencies that will be formed under such a scenario, given some number of routers n, is .5(n²—n). What this formula tells you is that as the number of routers sharing a broadcast link increases, the number of adjacencies increases exponentially.

Figure 4.22. Under the procedures described so far, all routers sharing ...