NHRP - Destination Protocol Address
NHRP is used to resolve the next hop IP address particularly in a DMVPN topology. This is achieved by exchanging NHRP Resolution Request and Resolution Reply messages.
Within these messages, there is a field called Destination Protocol Address or DPA. According to the RFC that defines NHRP (link below), this is:
…the protocol address of the station toward which a “request” packet is sent.
In other words, if you send a ping to 192.168.100.1 (the destination of the ICMP echo request) this is the address that appears in the field of the DPA. It is not the next hop address of the spoke, nor is it the tunneled address, but the destination of the actual transaction, whatever that may be. This is the case regardless of what phase DMVPN you use.
Links
https://forum.networklessons.com/t/dmvpn-phase-3-ospf-routing/1314/26?u=lagapides