The DMVPN design model consists of three phases.

Phase 1

In phase 1, the DMVPN spokes are registered with the hub. In this early phase, there is no direct communication between the spokes, so all traffic goes through the hub. Each spoke uses regular point-to-point GRE tunnel interfaces and requires only a summary or default route to the hub to reach other spokes. As a result, the routing configuration in this phase is simple.

Phase 2

This phase allows spoke-to-spoke tunnel deployment with all spoke routers using multipoint GRE tunnels. These spoke-to-spoke tunnels are on demand, i.e., triggered based on the spoke traffic. This means the data does not have to travel to a central hub first. While the hub is used for the control plane, it is not necessarily in the data plane. This key fact differentiates Phase 2 from Phase 1.

Phase 3

In phase 3, the spoke-to-spoke tunnels are deployed without using specific pre-made routes. To secure those routes on the fly, this phase uses NHRP traffic indication messages (redirect and shortcuts) from the hub. This phase improves the scalability of phase 2.

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