- A
The spoke routers are missing a static route for the remote spoke's internal network pointing to the mGRE tunnel interface.
Correct because without a route to the remote spoke's network via the tunnel, the spoke will send traffic to the hub, which may not forward it correctly, or the spoke may use a default route that does not use the tunnel.
- B
The NHRP authentication key is mismatched between the spokes.
Why wrong: Incorrect because NHRP authentication is only between spoke and hub; spokes do not authenticate directly with each other in phase 2.
- C
The tunnel key is not configured on the mGRE interface.
Why wrong: Incorrect because the tunnel key is used for identifying the tunnel, but the spokes can still communicate with the hub without it, and it would not prevent spoke-to-spoke traffic if routing is correct.
- D
The hub is not configured with 'ip nhrp redirect' and the spokes with 'ip nhrp shortcut'.
Why wrong: Incorrect because these commands are used for phase 3 DMVPN, not phase 2. In phase 2, spoke-to-spoke tunnels are initiated by NHRP requests without redirect.
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer is troubleshooting a DMVPN phase 2 deployment with IPv6 over mGRE tunnels. The spoke routers can ping the hub's tunnel IPv6 address, but cannot reach IPv6 networks behind other spokes. The engineer verifies that NHRP is configured and that the hub has a route to the spoke's internal networks. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The spoke routers are missing a static route for the remote spoke's internal network pointing to the mGRE tunnel interface.
In a DMVPN Phase 2 deployment, spoke routers must have a route to remote spoke networks pointing to the mGRE tunnel interface. Without this static route, the spoke will not know to send traffic for the remote spoke's internal network over the tunnel, even though NHRP resolves the next-hop. The hub has a route to the spoke's internal networks, but that does not enable direct spoke-to-spoke communication without proper routing on the spokes themselves.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The spoke routers are missing a static route for the remote spoke's internal network pointing to the mGRE tunnel interface.
Why this is correct
Correct because without a route to the remote spoke's network via the tunnel, the spoke will send traffic to the hub, which may not forward it correctly, or the spoke may use a default route that does not use the tunnel.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The NHRP authentication key is mismatched between the spokes.
Why it's wrong here
Incorrect because NHRP authentication is only between spoke and hub; spokes do not authenticate directly with each other in phase 2.
- ✗
The tunnel key is not configured on the mGRE interface.
Why it's wrong here
Incorrect because the tunnel key is used for identifying the tunnel, but the spokes can still communicate with the hub without it, and it would not prevent spoke-to-spoke traffic if routing is correct.
- ✗
The hub is not configured with 'ip nhrp redirect' and the spokes with 'ip nhrp shortcut'.
Why it's wrong here
Incorrect because these commands are used for phase 3 DMVPN, not phase 2. In phase 2, spoke-to-spoke tunnels are initiated by NHRP requests without redirect.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Phase 2 and Phase 3 DMVPN behavior, and the trap here is that candidates assume NHRP alone handles spoke-to-spoke routing, forgetting that a route pointing to the tunnel interface is required in Phase 2 for the spoke to initiate the NHRP resolution process.
Trap categories for this question
Command / output trap
Incorrect because these commands are used for phase 3 DMVPN, not phase 2. In phase 2, spoke-to-spoke tunnels are initiated by NHRP requests without redirect.
Detailed technical explanation
How to think about this question
In DMVPN Phase 2, the hub advertises routes for spoke networks, but spokes do not automatically install routes for remote spoke networks; they rely on a static route or a dynamic routing protocol to direct traffic to the tunnel interface. NHRP resolves the NBMA address of the destination spoke, but the IPv6 routing table must have a route pointing to the tunnel interface for the packet to be processed and NHRP resolution to occur. Without this route, the spoke will drop the packet or send it out the wrong interface.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The spoke routers are missing a static route for the remote spoke's internal network pointing to the mGRE tunnel interface. — In a DMVPN Phase 2 deployment, spoke routers must have a route to remote spoke networks pointing to the mGRE tunnel interface. Without this static route, the spoke will not know to send traffic for the remote spoke's internal network over the tunnel, even though NHRP resolves the next-hop. The hub has a route to the spoke's internal networks, but that does not enable direct spoke-to-spoke communication without proper routing on the spokes themselves.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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