When you starting talking about DMVPN you’ll typically hear it being described as a Phase I, II, or III type DMVPN network, so let’s quickly discuss the differences between these three DMVPN phases:
DMVPN Phase I: This phase involves configuring a single mGRE interface on the hub, and all the spokes are still static tunnels so you won’t get any dynamic spoke-to-spoke connectivity. The only advantage of the phase I setup is the fact the hub router’s configuration is much simpler.
DMVPN Phase II: This phase involves everysite being configured with mGRE interface so you get your dynamic spoke-to-spoke connectivity, no more static tunnel destination’s will be configured.
DMVPN Phase III: This phase expands on the scalability of the DMVPN network. This involve summarizing into the DMVPN cloud to provide (Remember EIGRP allows us to summarize out interfaces and BGP allows us to advertise aggregate addresses to neighbors). Along with configuring NHRP redirects and NHRP shortcut switching. NHRP redirects tells the source to find a better path to the destination it is trying to reach. NHRP shortcuts allow DMVPN to learn about other networks behind other DMVPN routers (kind of like ARP for DMVPN). However Phase III will get it’s own post later on.