NHRP - resolution redirect from hub

In a DMVPN topology, NHRP is used to determine the next hop for a particular destination. The NHRP resolution process differs somewhat depending upon the Phase that is configured.

When a spoke sends a resolution request to the hub for a particular destination spoke, the hub responds to the originator spoke with an NHRP reply providing the next hop IP information.

According to the RFC, the hub is not required to inform the destination spoke of this request. However, some vendors, like Cisco, choose to send an NHRP notification to the destination spoke as well, to let it know that another spoke has been informed of its NBMA IP address and may initiate direct communication. Although not a required part of the NHRP resolution process, it may be used to optimize the setup of the reverse path, facilitating the establishment of a bidirectional tunnel between the two spokes.

In a DMVPN topology, such a behavior only really has any value for Phase 3 setups. In Phase 1, spoke-to-spoke tunnels are never established so there is no need for the hub to inform the destination spoke that another spoke wants to communicate with it. Similarly in Phase 2, while the spokes do establish direct tunnels with each other, the NHRP resolution process is typically initiated by the spoke that wants to establish the tunnel. The hub replies to this with the necessary information to establish a direct tunnel to the destination spoke.