- A
R1 has no EIGRP configuration on the tunnel interface, so it does not send hellos over the tunnel.
EIGRP must be enabled on the tunnel interface to establish neighbor relationships over it.
- B
R2's NHRP registration is incomplete, so R1 cannot reach R2's NBMA address.
Why wrong: NHRP shows R2's NBMA address, so registration is complete.
- C
The hub R3 is not configured to forward EIGRP packets between spokes.
Why wrong: In DMVPN, spoke-to-spoke tunnels are direct; the hub does not forward EIGRP packets.
- D
R1's tunnel interface has an ACL that blocks EIGRP packets.
Why wrong: No ACL is mentioned.
Question 1,892 of 2,152
DMVPN Spoke-to-Spoke Tunnel Failure: EIGRP and NHRP
This 300-410 practice question tests your understanding of eigrp troubleshooting. 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 DMVPN network with EIGRP as the routing protocol is experiencing spoke-to-spoke tunnel failures. Spoke routers R1 and R2 are both connected to hub R3. R1 shows: 'show ip nhrp' lists R2's NBMA address, but 'show ip eigrp neighbors' does not show R2. R1's configuration: interface Tunnel0 ip nhrp network-id 1 ip nhrp nhs 10.0.0.3. What is the root cause?
Quick Answer
The answer is that R1 has no EIGRP configuration on the tunnel interface, so it does not send EIGRP hellos over the tunnel. In a DMVPN spoke-to-spoke tunnel failure scenario with EIGRP, NHRP successfully resolves the NBMA address of the remote spoke, as shown by the 'show ip nhrp' output, but without EIGRP enabled on the tunnel interface, no hello packets are generated to form a neighbor adjacency. This tests your understanding of the DMVPN control plane: NHRP handles dynamic address resolution, while EIGRP must be explicitly activated under the tunnel interface to establish and maintain spoke-to-spoke routing. On the Cisco CCNP ENARSI 300-410 exam, a common trap is assuming that a working NHRP mapping guarantees EIGRP neighbor formation—it does not. Remember the memory tip: “NHRP maps the path, but EIGRP must walk it.”
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
✓
R1 has no EIGRP configuration on the tunnel interface, so it does not send hellos over the tunnel.
R1's 'show ip nhrp' lists R2's NBMA address, confirming NHRP phase 2/3 spoke-to-spoke resolution is working. However, 'show ip eigrp neighbors' does not show R2, indicating EIGRP adjacency is not established. Since R1's tunnel interface lacks EIGRP configuration (no 'ip eigrp <as-number>' under Tunnel0), it does not send EIGRP hellos over the tunnel, preventing neighbor formation even though NHRP can dynamically map spoke addresses.
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.
- ✓
R1 has no EIGRP configuration on the tunnel interface, so it does not send hellos over the tunnel.
Why this is correct
EIGRP must be enabled on the tunnel interface to establish neighbor relationships over it.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
R2's NHRP registration is incomplete, so R1 cannot reach R2's NBMA address.
Why it's wrong here
NHRP shows R2's NBMA address, so registration is complete.
- ✗
The hub R3 is not configured to forward EIGRP packets between spokes.
Why it's wrong here
In DMVPN, spoke-to-spoke tunnels are direct; the hub does not forward EIGRP packets.
- ✗
R1's tunnel interface has an ACL that blocks EIGRP packets.
Why it's wrong here
No ACL is mentioned.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that NHRP and EIGRP are automatically linked; the trap here is that candidates assume a working NHRP mapping implies EIGRP adjacency, but EIGRP requires explicit interface-level configuration to send hellos over the tunnel.
Trap categories for this question
Command / output trap
NHRP shows R2's NBMA address, so registration is complete.
Detailed technical explanation
How to think about this question
EIGRP uses multicast (224.0.0.10) for hellos, which in a DMVPN requires the tunnel interface to be configured with 'ip eigrp <as>' to enable the protocol. Without this, the interface does not participate in EIGRP, so no hellos are sent or processed, even though NHRP can dynamically resolve spoke NBMA addresses. In real-world deployments, engineers often forget to apply EIGRP under the tunnel interface after configuring NHRP, leading to this exact symptom where NHRP shows mappings but EIGRP neighbors are absent.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: R1 has no EIGRP configuration on the tunnel interface, so it does not send hellos over the tunnel. — R1's 'show ip nhrp' lists R2's NBMA address, confirming NHRP phase 2/3 spoke-to-spoke resolution is working. However, 'show ip eigrp neighbors' does not show R2, indicating EIGRP adjacency is not established. Since R1's tunnel interface lacks EIGRP configuration (no 'ip eigrp <as-number>' under Tunnel0), it does not send EIGRP hellos over the tunnel, preventing neighbor formation even though NHRP can dynamically map spoke addresses.
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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