A network engineer deploys a Phase 2 DMVPN with a single hub and several spokes using EIGRP as the routing protocol. Spoke-to-spoke traffic is functional in some cases but fails in others. On a spoke, 'show ip route' displays remote spoke LAN subnets with the hub's tunnel IP as the next hop rather than the destination spoke's tunnel IP. What must be configured on the HUB tunnel interface to allow spokes to build direct spoke-to-spoke tunnels in this Phase 2 design?