mGRE

Regular GRE tunnels are configured in a point-to-point arrangement, where each tunnel has two endpoints. Multipoint GRE or mGRE as the name implies allows us to have multiple endpoints, resulting in a point to multipoint topology.

When we use mGRE, there will be only one tunnel interface on each router, which will connect with multiple other mGRE endpoints.

Links:

https://networklessons.com/cisco/ccie-enterprise-infrastructure/introduction-to-dmvpn#Multipoint_GRE