in DMVPN Phase-1 , after we configure this command on the spoke side
ip nhrp nhs {overlay ip on hub}
the spoke is going to register himself to the hub NHRP DB by sending (NHRP Registeration Request) message and then the hub send back ack message called (NHRP Registeration Reply)
then if spoke-1 need to communicate with spoke-2 , he need the NBMA address for the spoke-2, so spoke-1 send (NHRP Resolution Req)to the hub and the hub send BACK TO THE SPOKE-1 (NHRP Resolution Reply).
but in DMVPN Phase-2 something weird happened . when the spoke send (NHRP Resolution req)to the hub . what i saw is the hub is going to discard it and re-generate the same (NHRP Resolution req)with the source on himself and dst is spoke-2.
what is the secrect here that makes the hub side in phase-2 doing that is the configuration on the spoke side is the same except :
tunnel mode gre multipoint
ip nhrp map {overlay for hub}{underlay for the hub}
nothing fancy here is configured on the hub side which makes the spoke told the hub “hey, i`m in phase-2”