- A
The hub is missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication.
Without these commands, the hub does not trigger NHRP resolution for spoke-to-spoke traffic, and spokes do not install the necessary routes.
- B
The spoke routers have incorrect NHRP authentication, causing the mapping to be invalid.
Why wrong: Authentication mismatch would prevent NHRP registration, but the mapping exists.
- C
The tunnel interface on R1 is not in the correct VRF.
Why wrong: VRF misconfiguration would cause other issues, but not specifically spoke-to-spoke failure with existing NHRP mapping.
- D
The IPsec transform set is mismatched between spokes.
Why wrong: IPsec mismatch would prevent tunnel establishment, but the DMVPN session is up.
Quick Answer
The answer is that the hub is missing the `ip nhrp redirect` command and the spokes are missing `ip nhrp shortcut`, which prevents spoke-to-spoke direct communication. In a DMVPN Phase 3 network, when a spoke sends traffic to another spoke via the hub, the hub must send an NHRP redirect message to inform the source spoke of the destination spoke’s real physical address; the source spoke then uses the `ip nhrp shortcut` command to install a direct, on-demand tunnel. Without these commands, the hub forwards traffic between spokes without triggering the spoke-to-spoke tunnel, so even though R1 shows a dynamic NHRP mapping for R2, it lacks the shortcut flag and will not use that mapping for direct forwarding. This scenario tests your understanding of DMVPN Phase 3’s traffic flow optimization on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming a dynamic NHRP mapping alone enables spoke-to-spoke traffic. Remember: “Redirect on the hub, Shortcut on the spoke—without both, the direct path is broke.”
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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 DMVPN network with NHRP is configured for spoke-to-spoke tunnels. Spoke routers R1 and R2 are both connected to a hub router H1. Spoke-to-spoke traffic is not working. R1's show dmvpn shows a dynamic NHRP mapping for R2's tunnel IP to R2's physical IP, but ping from R1's tunnel IP to R2's tunnel IP fails. R1's show ip nhrp shows the mapping as 'dynamic' with no flags. The hub has no special configuration. What is the root cause?
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 missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication.
The correct answer is A because in a DMVPN phase 3 network, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to process them via the `ip nhrp shortcut` command. Without `ip nhrp redirect` on the hub, the hub forwards traffic between spokes without signaling them to establish a direct tunnel. The dynamic NHRP mapping on R1 for R2's tunnel IP indicates that R1 has learned R2's physical address via NHRP registration, but without the shortcut flag, R1 will not use that mapping to send traffic directly; instead, it continues to send traffic through the hub, which fails if the hub does not have a route or if the spoke-to-spoke tunnel is not triggered.
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 missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication.
Why this is correct
Without these commands, the hub does not trigger NHRP resolution for spoke-to-spoke traffic, and spokes do not install the necessary routes.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The spoke routers have incorrect NHRP authentication, causing the mapping to be invalid.
Why it's wrong here
Authentication mismatch would prevent NHRP registration, but the mapping exists.
- ✗
The tunnel interface on R1 is not in the correct VRF.
Why it's wrong here
VRF misconfiguration would cause other issues, but not specifically spoke-to-spoke failure with existing NHRP mapping.
- ✗
The IPsec transform set is mismatched between spokes.
Why it's wrong here
IPsec mismatch would prevent tunnel establishment, but the DMVPN session is up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between DMVPN phase 2 (where spokes automatically build direct tunnels without redirect) and phase 3 (which requires explicit redirect and shortcut commands), leading candidates to assume that a dynamic NHRP mapping alone is sufficient for spoke-to-spoke communication.
Detailed technical explanation
How to think about this question
In DMVPN phase 3, the hub uses NHRP redirect (RFC 2332) to inform spokes that a better path exists directly to another spoke. The spoke then sends an NHRP resolution request to the target spoke to build a direct tunnel. Without `ip nhrp shortcut` on the spokes, the spoke ignores the redirect and continues to forward traffic through the hub, which can cause packet loss if the hub does not have a route to the destination spoke's physical network. A real-world scenario is when spokes are in different subnets and the hub is the only router with a route to both; without redirect/shortcut, traffic hairpins through the hub, increasing latency and potentially causing MTU issues.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hub is missing the ip nhrp redirect command, and the spokes are missing ip nhrp shortcut, preventing spoke-to-spoke direct communication. — The correct answer is A because in a DMVPN phase 3 network, spoke-to-spoke traffic requires the hub to send NHRP redirect messages and the spokes to process them via the `ip nhrp shortcut` command. Without `ip nhrp redirect` on the hub, the hub forwards traffic between spokes without signaling them to establish a direct tunnel. The dynamic NHRP mapping on R1 for R2's tunnel IP indicates that R1 has learned R2's physical address via NHRP registration, but without the shortcut flag, R1 will not use that mapping to send traffic directly; instead, it continues to send traffic through the hub, which fails if the hub does not have a route or if the spoke-to-spoke tunnel is not triggered.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An engineer configures DMVPN Phase 2 with spoke-to-spoke tunnels. Spokes can ping each other's physical interfaces, but cannot establish a direct tunnel. NHRP registration is successful. Which is the most likely explanation?
hard- ✓ A.The hub is not configured with 'ip nhrp redirect' and the spokes are not configured with 'ip nhrp shortcut'.
- B.The spokes have different NHRP authentication strings, causing NHRP resolution to fail.
- C.The tunnel interface on the spokes is configured with 'tunnel mode gre multipoint' but the hub uses 'tunnel mode gre ip'.
- D.The spokes are using different IPsec transform sets, causing the IPsec tunnel to fail.
Why A: In DMVPN Phase 2, spoke-to-spoke tunnels require NHRP redirect and shortcut mechanisms to dynamically build direct tunnels. The hub must be configured with 'ip nhrp redirect' to send redirect messages to spokes, and spokes must have 'ip nhrp shortcut' to install the NHRP-learned /32 host routes for direct traffic. Without these, spokes will forward traffic through the hub even though they can ping each other's physical interfaces, preventing the establishment of a direct tunnel.
Last reviewed: Jun 24, 2026
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