Question 670 of 2,152

NAT and PAT →hardMultiple ChoiceObjective-mapped

Troubleshooting DMVPN Phase 2: Missing NHRP Redirect Prevents Spoke-to-Spoke Tunnels

This 300-410 practice question tests your understanding of nat and pat. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 routers can communicate with the hub, but spoke-to-spoke traffic does not trigger a direct tunnel. 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 router is missing the 'ip nhrp redirect' command.
Correct. Redirect is required for Phase 2 spoke-to-spoke.
B
The spoke routers have 'ip nhrp shortcut' configured.
Why wrong: Shortcut is required on spokes; its presence would help, not hinder.
C
The tunnel mode is set to gre multipoint on the spokes.
Why wrong: GRE multipoint is correct for Phase 2.
D
The NHRP authentication is mismatched.
Why wrong: Authentication mismatch would prevent all NHRP communication.

Full breakdown with real-world context →

Quick Answer

The answer is that the hub router is missing the 'ip nhrp redirect' command. In DMVPN Phase 2, spoke-to-spoke tunnels rely on the hub to send NHRP redirect messages when it forwards traffic between spokes; without this command, the hub never signals that a direct path exists, so the spoke never sends an NHRP resolution request to build a shortcut tunnel. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the NHRP redirect and shortcut handshake, a common trap being that students assume Phase 2 automatically creates spoke-to-spoke tunnels without explicit hub configuration. Remember the mnemonic: "Redirect from the hub, shortcut from the spoke" — if the hub doesn't redirect, the spoke won't shortcut.

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 router is missing the 'ip nhrp redirect' command.

In a DMVPN Phase 2 network, spoke-to-spoke tunnels are triggered dynamically when a spoke receives an NHRP redirect from the hub. The 'ip nhrp redirect' command on the hub router enables it to send redirect messages to spokes, informing them of a more direct path to another spoke. Without this command, the hub forwards traffic between spokes but never signals the spokes to build a direct tunnel, so spoke-to-spoke traffic continues to traverse the hub.

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 router is missing the 'ip nhrp redirect' command.
Why this is correct
Correct. Redirect is required for Phase 2 spoke-to-spoke.
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 spoke routers have 'ip nhrp shortcut' configured.
Why it's wrong here
Shortcut is required on spokes; its presence would help, not hinder.
✗
The tunnel mode is set to gre multipoint on the spokes.
Why it's wrong here
GRE multipoint is correct for Phase 2.
✗
The NHRP authentication is mismatched.
Why it's wrong here
Authentication mismatch would prevent all NHRP communication.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between Phase 2 and Phase 3 DMVPN behaviors, and the trap here is that candidates confuse the spoke's 'ip nhrp shortcut' (which is required for Phase 2) with the hub's 'ip nhrp redirect' (which is also required), assuming that only one of them is necessary for spoke-to-spoke tunnels.

Detailed technical explanation

How to think about this question

DMVPN Phase 2 relies on NHRP redirect messages (RFC 2332) sent by the hub to trigger spoke-to-spoke tunnel creation. The hub forwards the initial data packet between spokes and simultaneously sends an NHRP redirect to the source spoke, which then sends an NHRP resolution request to the destination spoke's NBMA address. Without 'ip nhrp redirect', the hub acts as a simple router, forwarding traffic without any NHRP signaling, leaving the spoke unaware of the direct path. In real-world deployments, missing this command is a common misconfiguration when migrating from Phase 1 to Phase 2.

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?

NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The hub router is missing the 'ip nhrp redirect' command. — In a DMVPN Phase 2 network, spoke-to-spoke tunnels are triggered dynamically when a spoke receives an NHRP redirect from the hub. The 'ip nhrp redirect' command on the hub router enables it to send redirect messages to spokes, informing them of a more direct path to another spoke. Without this command, the hub forwards traffic between spokes but never signals the spokes to build a direct tunnel, so spoke-to-spoke traffic continues to traverse the hub.

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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