Question 1,870 of 2,152

Device Management →hardMultiple ChoiceObjective-mapped

DMVPN Phase 2: Spoke-to-Spoke Tunnels Fail Without Route to Remote via Hub

This 300-410 practice question tests your understanding of device management. 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 configures a DMVPN Phase 2 network. Spoke-to-spoke tunnels do not form, even though NHRP registration is successful. Which is the most likely explanation?

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.

A
The spoke routers do not have a route to each other's tunnel IP via the hub, so the NHRP shortcut request fails.
Phase 2 requires that the spoke's routing table points to the hub for the remote spoke's tunnel IP; otherwise, the shortcut cannot be established.
B
The NHRP authentication key is mismatched between spokes.
Why wrong: NHRP authentication is between spoke and hub, not between spokes.
C
The mGRE interface is configured with tunnel mode gre multipoint on the spokes.
Why wrong: mGRE is required for Phase 2 and is correctly configured.
D
The hub is not configured with 'ip nhrp redirect' and the spokes with 'ip nhrp shortcut'.
Why wrong: These are required for Phase 2, but the scenario states registration is successful, implying these are configured.

Full breakdown with real-world context →

Quick Answer

The answer is that the spoke routers lack a route to each other's tunnel IP via the hub, which prevents the NHRP shortcut from being triggered. In DMVPN Phase 2, spoke-to-spoke tunnels rely on the NHRP redirect and shortcut mechanism, but this process only initiates when a spoke has a valid route in its routing table pointing to the remote spoke’s tunnel address through the hub. Without that specific route, the spoke cannot send the initial data packet to the hub for redirection, so the shortcut request never fires. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how Phase 2 differs from Phase 1 and Phase 3—the common trap is assuming that successful NHRP registration alone guarantees spoke-to-spoke connectivity. Remember the memory tip: “No route via hub, no shortcut—Phase 2 needs a path through the hub to kickstart the spoke-to-spoke tunnel.”

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 do not have a route to each other's tunnel IP via the hub, so the NHRP shortcut request fails.

In DMVPN Phase 2, spoke-to-spoke tunnels rely on NHRP shortcut requests. Even though NHRP registration succeeds, the spoke routers must have a route to each other's tunnel IP via the hub (typically a static route pointing to the hub's tunnel IP) for the NHRP shortcut request to be forwarded correctly. Without this route, the spoke cannot reach the destination spoke's tunnel IP through the hub, so the NHRP Resolution Request fails, and the direct tunnel never forms.

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 do not have a route to each other's tunnel IP via the hub, so the NHRP shortcut request fails.
Why this is correct
Phase 2 requires that the spoke's routing table points to the hub for the remote spoke's tunnel IP; otherwise, the shortcut cannot be established.
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 spokes.
Why it's wrong here
NHRP authentication is between spoke and hub, not between spokes.
✗
The mGRE interface is configured with tunnel mode gre multipoint on the spokes.
Why it's wrong here
mGRE is required for Phase 2 and is correctly configured.
✗
The hub is not configured with 'ip nhrp redirect' and the spokes with 'ip nhrp shortcut'.
Why it's wrong here
These are required for Phase 2, but the scenario states registration is successful, implying these are configured.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between DMVPN Phase 2 and Phase 3, and the trap here is that candidates mistakenly think 'ip nhrp redirect' and 'ip nhrp shortcut' are required for spoke-to-spoke tunnels in Phase 2, when in fact they are Phase 3 features and the real issue is the missing route to the remote spoke's tunnel IP via the hub.

Trap categories for this question

Scenario analysis trap
These are required for Phase 2, but the scenario states registration is successful, implying these are configured.

Detailed technical explanation

How to think about this question

In DMVPN Phase 2, the spoke's routing table must have a route to the remote spoke's tunnel IP via the hub's tunnel IP (e.g., ip route 10.0.0.2 255.255.255.255 10.0.0.1) so that the NHRP Resolution Request packet is forwarded to the hub, which then relays it to the target spoke. Without this route, the spoke sends the request out its physical interface instead of the tunnel, causing failure. This is a common misconfiguration because engineers often assume NHRP registration alone ensures reachability, but NHRP operates over the tunnel interface and requires proper routing for control traffic.

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?

Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The spoke routers do not have a route to each other's tunnel IP via the hub, so the NHRP shortcut request fails. — In DMVPN Phase 2, spoke-to-spoke tunnels rely on NHRP shortcut requests. Even though NHRP registration succeeds, the spoke routers must have a route to each other's tunnel IP via the hub (typically a static route pointing to the hub's tunnel IP) for the NHRP shortcut request to be forwarded correctly. Without this route, the spoke cannot reach the destination spoke's tunnel IP through the hub, so the NHRP Resolution Request fails, and the direct tunnel never forms.

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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