The Generic Routing Encapsulation (GRE) protocol provides a simple general-purpose mechanism for encapsulating packets from a wide variety of network protocols to be forwarded over another protocol. In DMVPN, GRE encapsulates IP packets and transports them over VPN tunnels. An example is multicast routing advertisements, which are multicast. IPsec, which is a standard mechanism for providing security on IP networks, cannot encrypt multicast packets. However, multicast packets can be encapsulated within a GRE tunnel and then routed over a VPN connection, so that the encapsulated packets are protected by the IPsec tunnel.
Multipoint GRE (mGRE) allows an interface to support multiple GRE tunnels. In a DMVPN, multipoint mGRE tunnels are used to establish and aggregate the tunnels from the spokes to the hub.
mGRE commands are described in Ciena Vyatta Network OS Tunnels Configuration Guide.