300-440 Question 172
Single answerAn organization is deploying a DMVPN solution with a single hub. The hub router is configured with a static public IP address, while the spoke routers are located behind NAT devices with dynamic public IPs. After completing the DMVPN configuration, the spoke routers fail to establish a tunnel with the hub. What is the most likely cause of the issue?
- A
The hub router is missing the
ip nhrp map multicast dynamiccommand. - B
The spoke routers have not been configured with the
ip nhrp network-idcommand. - C
The hub router does not have the
ip nhrp redirectcommand configured. - D
The spoke routers do not have the
ip nhrp nhscommand pointing to the hub.
Show answer and explanation
Correct answer: A
Explanation
In a single-hub DMVPN deployment where the spokes are behind NAT devices, the ip nhrp map multicast dynamic command on the hub router ensures that multicast traffic is dynamically mapped to allow NAT traversal. Without this command, the spoke routers cannot establish a tunnel with the hub, resulting in communication failure. Other commands mentioned, while important, either do not address the specific NAT scenario or are unrelated to the issue described.
- A. Correct.
The
ip nhrp map multicast dynamiccommand is required on the hub router to enable dynamic mapping of multicast traffic, which is essential for spokes behind NAT to communicate with the hub. Without this command, the spokes cannot establish tunnels. - B. Incorrect.
The
ip nhrp network-idcommand is necessary on both the hub and spokes to identify the DMVPN domain, but its absence would not specifically cause the issue of failing to establish a tunnel with NAT devices. - C. Incorrect.
The
ip nhrp redirectcommand is used to optimize spoke-to-spoke traffic in DMVPN Phase 3 deployments. Since this is a single-hub DMVPN scenario, this command is not relevant to the issue described. - D. Incorrect.
The
ip nhrp nhscommand is mandatory on the spoke routers to designate the hub as the Next Hop Server. However, if this command were missing, the spoke routers would fail to identify the hub altogether, which is not the issue described in the scenario.