- A
The hub is missing the 'ip nhrp redirect' command under the tunnel interface.
Without NHRP redirect, the hub does not inform spokes of peer public addresses, preventing dynamic spoke-to-spoke tunnel formation.
- B
The IPsec profile on the hub does not include perfect forward secrecy (PFS).
Why wrong: PFS is not required for spoke-to-spoke tunnels; it affects key derivation but not NHRP redirect.
- C
The spokes are using different ISAKMP policies.
Why wrong: Different ISAKMP policies would prevent IKE phase 1, but the hub-to-spoke tunnels work, so ISAKMP is consistent.
- D
The NAT traversal feature is disabled on the hub.
Why wrong: NAT-T is needed for spokes behind NAT, but the hub-to-spoke tunnels work; missing NAT-T would break all tunnels, not just spoke-to-spoke.
Quick Answer
The answer is that the hub is missing the 'ip nhrp redirect' command under its tunnel interface. In a DMVPN Phase 2 topology, spoke-to-spoke dynamic tunnels rely on the hub to send NHRP redirect messages; without this command, the hub forwards traffic between spokes but never signals the source spoke to initiate a direct tunnel. This explains why R3 can ping R4’s tunnel IP via the hub, yet no NHRP redirect or shortcut appears, and no IPsec SA is built between the spokes. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the Phase 2 control-plane difference from Phase 3—a common trap is assuming spoke-to-spoke tunnels form automatically once IPsec is configured. Remember the mnemonic: “Phase 2 needs a redirect from the hub to you.”
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 large enterprise is using a DMVPN Phase 2 hub-and-spoke topology with IPsec protection. Spoke routers R3 and R4 are both behind NAT. The hub R1 has a tunnel interface with IPsec profile and mGRE. Spoke-to-spoke dynamic tunnels do not form. R3 can ping R4's tunnel IP via the hub, but R3's show dmvpn detail shows no NHRP redirect or shortcut. R4's show crypto ipsec sa shows no inbound/outbound SA for the R3-to-R4 traffic. 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 under the tunnel interface.
In a DMVPN Phase 2 topology, spoke-to-spoke dynamic tunnels require the hub to send NHRP redirect messages to inform a spoke that the destination it is trying to reach is behind another spoke. Without the 'ip nhrp redirect' command on the hub's tunnel interface, the hub forwards traffic between spokes but never signals the source spoke to initiate a direct tunnel. This explains why R3 can ping R4's tunnel IP via the hub but no NHRP redirect or shortcut is seen, and no IPsec SA is built between the spokes.
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 under the tunnel interface.
Why this is correct
Without NHRP redirect, the hub does not inform spokes of peer public addresses, preventing dynamic spoke-to-spoke tunnel formation.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The IPsec profile on the hub does not include perfect forward secrecy (PFS).
Why it's wrong here
PFS is not required for spoke-to-spoke tunnels; it affects key derivation but not NHRP redirect.
- ✗
The spokes are using different ISAKMP policies.
Why it's wrong here
Different ISAKMP policies would prevent IKE phase 1, but the hub-to-spoke tunnels work, so ISAKMP is consistent.
- ✗
The NAT traversal feature is disabled on the hub.
Why it's wrong here
NAT-T is needed for spokes behind NAT, but the hub-to-spoke tunnels work; missing NAT-T would break all tunnels, not just spoke-to-spoke.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between DMVPN Phase 2 and Phase 3, where Phase 3 uses 'ip nhrp redirect' on the hub and 'ip nhrp shortcut' on spokes, and candidates may incorrectly assume that spoke-to-spoke tunnels form automatically without explicit NHRP redirect configuration.
Detailed technical explanation
How to think about this question
NHRP redirect is a critical mechanism in DMVPN Phase 2 that allows the hub to inform a spoke that the destination IP address is reachable via another spoke, triggering the source spoke to send an NHRP resolution request to the destination spoke. The destination spoke then responds with its real (non-NATed) address, allowing the source spoke to initiate a direct IPsec tunnel. Without 'ip nhrp redirect', the hub acts as a simple forwarder and never provides the necessary routing hint, so spoke-to-spoke tunnels never form even though the hub can forward 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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — 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 under the tunnel interface. — In a DMVPN Phase 2 topology, spoke-to-spoke dynamic tunnels require the hub to send NHRP redirect messages to inform a spoke that the destination it is trying to reach is behind another spoke. Without the 'ip nhrp redirect' command on the hub's tunnel interface, the hub forwards traffic between spokes but never signals the source spoke to initiate a direct tunnel. This explains why R3 can ping R4's tunnel IP via the hub but no NHRP redirect or shortcut is seen, and no IPsec SA is built between the spokes.
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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Last reviewed: Jun 24, 2026
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