Which parameter must match on all routers in a DMVPN Phase 3 cloud?

The GRE tunnel key is used to identify and demultiplex GRE packets on a shared tunnel interface. In DMVPN Phase 3, all routers share a common multipoint GRE interface, so the tunnel key must match on every spoke and hub to correctly associate incoming packets with the right tunnel. A mismatch causes the tunnel to drop packets silently.

The NHRP network ID is locally significant to each router and does not need to match across all routers in the DMVPN cloud.

Tunnel VRF is an optional configuration used to separate the transport network and does not need to be uniform across all Phase 3 routers.

EIGRP split-horizon is disabled by default on mGRE tunnel interfaces and its setting does not need to be identical across all routers for Phase 3 to function.