NHRP redirect is a function that allows the hub to inform the source spoke of a better path to reach the destination spoke, by sending an NHRP redirect message containing the IP address of the destination spoke. This triggers the source spoke to send an NHRP resolution request to the destination spoke, in order to establish a direct spoke-to-spoke tunnel1.
NHRP resolution reply is a function that allows the destination spoke to respond to the NHRP resolution request from the source spoke, by sending an NHRP resolution reply containing its own IP address and the IP address of the source spoke. This confirms the establishment of the direct spoke-to-spoke tunnel, and also allows the destination spoke to create a reciprocal tunnel to the source spoke2.
These two functions are specific to DMVPN Phase 3, because they enable spoke-to-spoke communication without requiring a dynamic routing protocol or going through the hub. In DMVPN Phase 1 and Phase 2, NHRP registration request, registration reply, and resolution request are also used, but they have different purposes and effects3.