Question

A network engineer configures a single-hub DMVPN. On a spoke router, the tunnel interface uses 'tunnel source GigabitEthernet0/1' and 'ip nhrp nhs 172.16.0.1'. The spoke's tunnel line protocol is up, but 'show ip nhrp' on the hub shows no dynamic entry for this spoke, and 'show dmvpn' on the spoke lists the NHS state as 'NHRP'. The physical WAN interface Gi0/1 has connectivity to the hub's public address, and IPsec is not configured. What is the most likely cause of the failed registration?

Accepted Answer

The spoke's 'ip nhrp map' statement for the hub is missing, so the spoke cannot resolve the hub's NBMA address to send registration requests

Answer

The hub must be configured with 'ip nhrp nhs' pointing back to each spoke for bidirectional registration

Answer

The tunnel mode is set to 'tunnel mode gre multipoint' on the spoke, which prevents Phase 1 registration

Answer

The spoke and hub tunnel interfaces are in different IP subnets, which is required for DMVPN registration