7.5. The graph of adjacencies

An adjacency is bound to the network that the two routers have in common. If two routers have multiple networks in common, they may have multiple adjacencies between them.

One can picture the collection of adjacencies on a network as forming an undirected graph. The vertices consist of routers, with an edge joining two routers if they are adjacent. The graph of adjacencies describes the flow of routing protocol packets, and in particular Link State Update Packets, through the Autonomous System.

Two graphs are possible, depending on whether the common network is multi-access. On physical point-to-point networks (and virtual links), the two routers joined by the network will be adjacent after their databases have been synchronized. On multi-access networks, both the Designated Router and the Backup Designated Router are adjacent to all other routers attached to the network, and these account for all adjacencies.

These graphs are shown in Figure 10. It is assumed that Router RT7 has become the Designated Router, and Router RT3 the Backup Designated Router, for the Network N2. The Backup Designated Router performs a lesser function during the flooding procedure than the Designated Router (see Section 13.3). This is the reason for the dashed lines connecting the Backup Designated Router RT3.