Question 132 of 2,152

Route Redistribution →hardMultiple ChoiceObjective-mapped

Troubleshooting DMVPN Phase 2: Why Spoke-to-Spoke Traffic Fails

This 300-410 practice question tests your understanding of route redistribution. 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.

A network engineer configures DMVPN Phase 2 with NHRP. Spoke routers can communicate with the hub, but spoke-to-spoke traffic fails. The 'show dmvpn' on spokes shows the hub's NBMA address but not other spokes. 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 hub is configured with 'ip nhrp redirect' but spokes are missing 'ip nhrp shortcut'.
Without 'ip nhrp shortcut' on spokes, they will not install dynamic NHRP entries.
B
The tunnel interface mode is GRE IP, not mGRE.
Why wrong: DMVPN uses mGRE; GRE IP would prevent multiple destinations.
C
The spokes have different IPsec transform sets configured.
Why wrong: IPsec mismatch would prevent tunnel establishment entirely.
D
The NHRP authentication string is missing on the hub.
Why wrong: NHRP auth mismatch would prevent registration, not spoke-to-spoke after registration.

Full breakdown with real-world context →

Quick Answer

The answer is that the hub is configured with 'ip nhrp redirect' but the spokes are missing 'ip nhrp shortcut'. In DMVPN Phase 2, spoke-to-spoke traffic relies on the hub sending NHRP redirect messages to inform a spoke that a better path exists directly to another spoke, and the receiving spoke must have the 'ip nhrp shortcut' command enabled to dynamically install that direct tunnel. Without the shortcut on the spokes, they ignore the redirect, so they never learn each other’s NBMA addresses and continue forwarding traffic through the hub, which explains why 'show dmvpn' only shows the hub. This scenario directly tests your understanding of the NHRP redirect/shortcut mechanism, a frequent trap on the Cisco CCNP ENARSI 300-410 exam where candidates mistakenly focus on tunnel mode or authentication. Remember the mnemonic: "The hub redirects, the spoke shortcuts"—both must be present for Phase 2 spoke-to-spoke to work.

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 hub is configured with 'ip nhrp redirect' but spokes are missing 'ip nhrp shortcut'.

In DMVPN Phase 2, spoke-to-spoke tunnels require NHRP redirect from the hub and NHRP shortcut on spokes. The hub is correctly configured with 'ip nhrp redirect', which sends redirect messages to spokes when traffic destined for another spoke passes through the hub. However, if spokes are missing 'ip nhrp shortcut', they will ignore the redirect messages and not install a direct NHRP shortcut path. Consequently, spoke-to-spoke traffic continues to be forwarded via the hub, and the 'show dmvpn' output on spokes will not show other spokes' NBMA addresses because no shortcut sessions are established.

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 hub is configured with 'ip nhrp redirect' but spokes are missing 'ip nhrp shortcut'.
Why this is correct
Without 'ip nhrp shortcut' on spokes, they will not install dynamic NHRP entries.
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 tunnel interface mode is GRE IP, not mGRE.
Why it's wrong here
DMVPN uses mGRE; GRE IP would prevent multiple destinations.
✗
The spokes have different IPsec transform sets configured.
Why it's wrong here
IPsec mismatch would prevent tunnel establishment entirely.
✗
The NHRP authentication string is missing on the hub.
Why it's wrong here
NHRP auth mismatch would prevent registration, not spoke-to-spoke after registration.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Route Redistribution — This question tests Route Redistribution — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The hub is configured with 'ip nhrp redirect' but spokes are missing 'ip nhrp shortcut'. — In DMVPN Phase 2, spoke-to-spoke tunnels require NHRP redirect from the hub and NHRP shortcut on spokes. The hub is correctly configured with 'ip nhrp redirect', which sends redirect messages to spokes when traffic destined for another spoke passes through the hub. However, if spokes are missing 'ip nhrp shortcut', they will ignore the redirect messages and not install a direct NHRP shortcut path. Consequently, spoke-to-spoke traffic continues to be forwarded via the hub, and the 'show dmvpn' output on spokes will not show other spokes' NBMA addresses because no shortcut sessions are established.

What should I do if I get this 300-410 question wrong?

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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