The answer is the spoke’s tunnel interface is missing the `ip nhrp map multicast dynamic` command. In DMVPN Phase 2, EIGRP relies on multicast hello packets to form neighbor relationships, and without a multicast map on the spoke, the hub cannot receive these packets even though unicast pings succeed. This is because NHRP controls how multicast traffic is forwarded over the mGRE tunnel; the spoke must explicitly map the hub’s NBMA address to multicast, either statically or dynamically, to allow the hub to process EIGRP hellos. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of DMVPN multicast requirements and the difference between unicast reachability and multicast forwarding—a common trap is assuming a successful ping confirms full tunnel functionality. Remember the memory tip: “Ping works, but no neighbor? Check the multicast map—dynamic or static, it’s the spoke’s ticket to talk.”
300-410 DMVPN Practice Question
This 300-410 practice question tests your understanding of dmvpn. 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.
An engineer is troubleshooting a DMVPN phase 2 network where the hub router is not forming an EIGRP neighbor relationship with a spoke. The spoke's tunnel interface is configured with 'ip nhrp nhs 10.0.0.1' and 'ip nhrp map 10.0.0.1 192.168.1.1'. The hub's tunnel interface IP is 10.0.0.1. The engineer pings the hub's tunnel IP from the spoke and it succeeds. The engineer checks 'show ip eigrp neighbors' on the hub and sees no neighbors. What is the most likely cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The spoke's tunnel interface is missing the 'ip nhrp map multicast dynamic' command.
EIGRP neighbor formation over a tunnel requires multicast support. In DMVPN, multicast traffic is sent via NHRP to the hub. If the spoke's tunnel interface does not have 'ip nhrp map multicast dynamic' or a static multicast map, the hub will not receive EIGRP hello packets from the spoke.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 spoke's tunnel interface is missing the 'ip nhrp map multicast dynamic' command.
Why this is correct
Correct because without multicast mapping, the spoke cannot send multicast traffic (including EIGRP hellos) to the hub.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The hub's tunnel interface has 'no ip nhrp redirect' configured.
Why it's wrong here
Incorrect because redirect is for spoke-to-spoke traffic, not for multicast.
✗
The spoke's EIGRP AS number does not match the hub's.
Why it's wrong here
Incorrect because AS mismatch would prevent neighbor formation, but the engineer would check that first.
✗
The hub's tunnel interface has 'ip nhrp authentication' configured but the spoke does not.
Why it's wrong here
Incorrect because NHRP authentication affects NHRP registration, not EIGRP neighbor formation.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
DMVPN — This question tests DMVPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The spoke's tunnel interface is missing the 'ip nhrp map multicast dynamic' command. — EIGRP neighbor formation over a tunnel requires multicast support. In DMVPN, multicast traffic is sent via NHRP to the hub. If the spoke's tunnel interface does not have 'ip nhrp map multicast dynamic' or a static multicast map, the hub will not receive EIGRP hello packets from the spoke.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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