Which protocol resolves the NBMA (underlay) address of the remote endpoint for mGRE tunnels?

NHRP (Next Hop Resolution Protocol) is crucial for mGRE (multipoint Generic Routing Encapsulation) tunnels because it dynamically maps the private IP addresses of spoke routers to their public, routable NBMA (Non-Broadcast Multi-Access) addresses. This allows spokes to communicate directly with each other without needing to traverse the hub, improving efficiency and reducing latency. Without NHRP, spokes would only know the hub's NBMA address and would have to forward all traffic through the hub, defeating the purpose of a multipoint tunnel. IPsec provides encryption and authentication but doesn't resolve addresses. MP-BGP is a routing protocol for exchanging routing information, not for address resolution in this context. OSPF is an interior gateway routing protocol and does not perform NBMA address resolution for tunnels.