GRE, by default, is a point-to-point virtual private network that requires configuration between each pair of routers that you want to transmit traffic between. mGRE overcomes this limitation.
mGRE and NHRP is the pairing which provides the basis for building dynamic tunnels in hub and spoke environments. On the hub router, mGRE enables a single GRE interface to service the multiple GRE tunnels between spoke sites.
Like GRE, mGRE also supports unicast, broadcast, and multicast traffic. (Both also provide support for non-IP protocols.) A key characteristic of mGRE is the ability to define one tunnel interface in support of multiple remote peers.
All DMVPN members will use either GRE or mGRE as illustrated in the two diagrams below.
In Figure 1 above, the hub router is configured with mGRE and the spoke sites are configured with GRE. This enables a traditional hub-and-spoke architecture where the spoke sites are not able to route directly with others, and all traffic traverses the hub site. Configuration is still quite manageable, and NHRP configuration on the hub router allows it to learn the addressing of the remote sites if it is dynamic.
In Figure 2 above, the hub router is configured with mGRE and so are the spoke sites. This enables a full mesh, where both the hub and spokes are able to use NHRP for dynamic tunnel initiation and dynamic address support. Traffic does not have to traverse the hub if the optimal routing path is directly between spoke sites.
Stuck-in-Active: Journal of an IT-Network Administrator 