- A
The spokes are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses.
Without NHS configuration, spokes do not register with the hub, so NHRP cannot provide the mapping for spoke-to-spoke tunnels.
- B
The tunnel mode is mGRE, but the spokes are configured with point-to-point GRE tunnels.
Why wrong: Spokes typically use mGRE as well; if they used p2p GRE, they would only connect to the hub, but the scenario does not indicate this.
- C
The NHRP network-id is different on R2 and R3, preventing communication.
Why wrong: The network-id must match, but the scenario shows it is 1 on the hub; if spokes had different IDs, they would not register.
- D
The tunnel source interface is not reachable between spokes.
Why wrong: Spoke-to-spoke reachability is established via NHRP; if NHRP is not working, the tunnel source may be reachable but the tunnel itself fails.
DMVPN Spoke-to-Spoke Tunnels Down: NHRP NHS Not Configured on Spokes
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.
A DMVPN network with hub R1 and spokes R2 and R3 is configured with mGRE and NHRP. Spoke-to-spoke tunnels fail to form. R1 configuration: interface Tunnel0, ip address 10.0.0.1 255.255.255.0, tunnel source GigabitEthernet0/0, tunnel mode gre multipoint, ip nhrp network-id 1, ip nhrp map multicast dynamic. R2 shows: 'show dmvpn' shows no dynamic sessions. R3 shows: 'show ip nhrp' shows no entries for R2. What is the root cause?
Quick Answer
The answer is that the spokes are missing the 'ip nhrp nhs' command pointing to the hub, which prevents NHRP registration and breaks spoke-to-spoke tunnel formation. Without this command, R2 and R3 never register their real (physical) addresses with the hub, so the hub cannot relay those addresses to other spokes via NHRP resolution replies. When a spoke tries to build a direct tunnel to another spoke, it has no NHRP mapping to resolve—hence the 'show dmvpn' shows no dynamic sessions and 'show ip nhrp' shows no entries for the peer. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap: candidates often focus on the hub's 'ip nhrp map multicast dynamic' and assume spokes will automatically register, but the spokes must explicitly declare the hub as their NHS. A common memory tip is "Spokes need NHS to speak"—without the NHS command, the spoke never introduces itself, and the DMVPN cloud stays silent.
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 spokes are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses.
The correct answer is A because in a DMVPN network, spokes must use the 'ip nhrp nhs' command to register their real (physical) IP addresses with the hub (NHS). Without this command, the hub never learns the spokes' addresses, so it cannot provide NHRP resolution replies to other spokes attempting to form spoke-to-spoke tunnels. The absence of dynamic sessions on R2 and no NHRP entries for R2 on R3 directly indicate that R2 never registered with 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 spokes are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses.
Why this is correct
Without NHS configuration, spokes do not register with the hub, so NHRP cannot provide the mapping for spoke-to-spoke tunnels.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The tunnel mode is mGRE, but the spokes are configured with point-to-point GRE tunnels.
Why it's wrong here
Spokes typically use mGRE as well; if they used p2p GRE, they would only connect to the hub, but the scenario does not indicate this.
- ✗
The NHRP network-id is different on R2 and R3, preventing communication.
Why it's wrong here
The network-id must match, but the scenario shows it is 1 on the hub; if spokes had different IDs, they would not register.
- ✗
The tunnel source interface is not reachable between spokes.
Why it's wrong here
Spoke-to-spoke reachability is established via NHRP; if NHRP is not working, the tunnel source may be reachable but the tunnel itself fails.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'ip nhrp map multicast dynamic' alone is sufficient for spoke registration, but in reality, the 'ip nhrp nhs' command is mandatory for spokes to register their addresses with the hub.
Trap categories for this question
Command / output trap
The network-id must match, but the scenario shows it is 1 on the hub; if spokes had different IDs, they would not register.
Scenario analysis trap
Spokes typically use mGRE as well; if they used p2p GRE, they would only connect to the hub, but the scenario does not indicate this.
Detailed technical explanation
How to think about this question
NHRP (Next Hop Resolution Protocol) relies on a Next Hop Server (NHS) to maintain a database of registered clients. The 'ip nhrp nhs' command on a spoke designates the hub as the NHS, triggering periodic registration packets that include the spoke's NBMA (physical) and tunnel (virtual) addresses. Without this, the hub's NHRP cache remains empty for that spoke, so when another spoke queries for the destination tunnel IP, the hub cannot respond with the required NBMA address, breaking spoke-to-spoke dynamic tunnel establishment.
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
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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 spokes are missing the 'ip nhrp nhs' command to point to the hub, preventing registration and resolution of spoke-to-spoke addresses. — The correct answer is A because in a DMVPN network, spokes must use the 'ip nhrp nhs' command to register their real (physical) IP addresses with the hub (NHS). Without this command, the hub never learns the spokes' addresses, so it cannot provide NHRP resolution replies to other spokes attempting to form spoke-to-spoke tunnels. The absence of dynamic sessions on R2 and no NHRP entries for R2 on R3 directly indicate that R2 never registered with 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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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