Question

A network engineer is configuring a single-hub DMVPN. The hub tunnel interface is up, but a newly added spoke shows no NHRP registration on the hub (show ip nhrp lists no entries for the spoke). The spoke's tunnel interface is up/up and the underlying physical path to the hub NBMA address is reachable via ping. On the spoke, the relevant configuration is:

interface Tunnel0
 ip address 10.0.0.2 255.255.255.0
 tunnel source GigabitEthernet0/0
 tunnel mode gre multipoint
 ip nhrp network-id 100
 ip nhrp nhs 10.0.0.1

The hub uses 'ip nhrp map multicast dynamic' and 'ip nhrp network-id 100'. What is the most likely cause the spoke fails to register with the hub?

Accepted Answer

The spoke is missing the 'ip nhrp map 10.0.0.1 <hub-NBMA-address>' statement, so it cannot resolve where to send the NHRP registration request.

Answer

The hub must also configure 'ip nhrp nhs' pointing back at each spoke tunnel address before registration can complete.

Answer

The spoke tunnel must use 'tunnel mode gre' (point-to-point) instead of multipoint to register with a single hub.

Answer

The network-id values must be globally unique per router, so the spoke should use 'ip nhrp network-id 200'.