Which protocol is used to determine the NBMA address on the other end of a tunnel when mGRE is used?

NHRP provides a mechanism for dynamic discovery of NBMA addresses for spokes in an mGRE (multipoint GRE) tunnel. It allows spoke routers to register their public NBMA address and private tunnel IP address with a hub, and then query the hub to find the NBMA address of other spokes when direct spoke-to-spoke communication is desired.

IPsec is a suite of protocols used for secure communication (authentication, encryption) over an IP network, but it is not used for NBMA address resolution in mGRE.

MP-BGP (Multiprotocol BGP) is an extension of BGP that carries routing information for multiple network layer protocols, including VPNv4 for MPLS VPNs, but it's a routing protocol, not an NBMA address resolution protocol.

OSPF (Open Shortest Path First) is an interior gateway routing protocol (IGP) that establishes and maintains routing tables, but it does not perform NBMA address resolution for mGRE tunnels.