300-440 Question 179
Single answerIn a DMVPN Phase 2 network, you are configuring NHRP on a spoke router. The spoke router is not able to communicate with another spoke router via the tunnel interface. Upon investigation, you find that the spoke is properly registering with the hub, but the packets between spokes are still being forwarded through the hub instead of directly between the spokes. What could be the cause of this issue?
- A
The spoke router is configured with 'ip nhrp redirect', which is unnecessary in a Phase 2 deployment.
- B
The hub router is missing the 'ip nhrp redirect' command, which is required for direct spoke-to-spoke communication.
- C
The spoke routers are missing the 'ip nhrp shortcut' command, which is required for direct spoke-to-spoke communication in Phase 2.
- D
The DMVPN is misconfigured as Phase 3 instead of Phase 2, preventing direct spoke-to-spoke communication.
Show answer and explanation
Correct answer: C
Explanation
In DMVPN Phase 2, the hub facilitates the initial NHRP registration and mapping, but for direct spoke-to-spoke communication, the spoke routers must have the 'ip nhrp shortcut' command configured. Without this command, the spokes cannot resolve the next-hop mappings directly and will continue to forward packets through the hub, even when direct communication is possible. This issue is specific to Phase 2 and does not involve the 'ip nhrp redirect' command, which is used in Phase 3.
- A. Incorrect.
This is incorrect. The 'ip nhrp redirect' command is used on the hub in a DMVPN Phase 3 setup, but it is not relevant in Phase 2. Configuring it on the spoke is unnecessary.
- B. Incorrect.
This is incorrect. The 'ip nhrp redirect' command is specific to the hub in Phase 3 and is not required in a Phase 2 deployment.
- C. Correct.
This is correct. In DMVPN Phase 2, spoke routers require the 'ip nhrp shortcut' command to resolve the NHRP mappings and enable direct spoke-to-spoke communication.
- D. Incorrect.
This is incorrect. The question specifies that the DMVPN is configured as Phase 2. Misconfiguring it as Phase 3 would result in a different set of issues, but that is not the cause here.