DMVPNs require that routers run Next Hop Resolution Protocol (NHRP), which uses a client-server model. A router designated as a hub router acts as a server. The remaining routers, designated as spokes, act as clients. NHRP spokes are configured with the IP address of the NHRP hub, and when a spoke comes online, it informs the hub of both a physical IP address (assigned to its physical interface) and a logical IP address (assigned to its virtual tunnel interface) that are going to be used for its tunnels.
As an example, examine Figure 2-8.