The answer is an NHRP authentication mismatch, specifically that the hub router has 'ip nhrp authentication DMVPN' configured while the spoke does not. This is correct because NHRP registration in DMVPN Phase 2 requires the spoke to send a Registration Request to the hub’s NHS; if the hub’s authentication string is missing or mismatched on the spoke, the hub silently drops the request, preventing any NHRP entries from appearing in the spoke’s table—even though a ping to the hub’s tunnel IP succeeds, since ICMP operates at Layer 3 and bypasses NHRP authentication. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of DMVPN control-plane vs. data-plane behavior: a successful ping confirms the tunnel is up, but a missing NHRP adjacency points directly to authentication or NHS configuration errors. A common trap is assuming a reachable tunnel IP guarantees NHRP registration, so remember that NHRP authentication must match exactly on both hub and spoke for registration to complete. Memory tip: “Ping passes, NHRP fails? Check the auth string—it’s the silent killer of DMVPN Phase 2.”
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.
A network engineer is troubleshooting a DMVPN phase 2 network where the hub router is not forming an NHRP adjacency with a spoke. The spoke router 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, and the physical interface IP is 192.168.1.1. The engineer pings the hub's tunnel IP from the spoke and it succeeds. However, 'show ip nhrp' on the spoke shows no NHRP entries. 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 hub router has 'ip nhrp authentication DMVPN' configured, but the spoke does not.
NHRP registration requires the spoke to send a Registration Request to the hub. If the hub does not respond, the spoke will not have NHRP entries. A common cause is that the hub's NHRP authentication is configured with a password, but the spoke's NHRP authentication is missing or mismatched.
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 hub router has 'ip nhrp authentication DMVPN' configured, but the spoke does not.
Why this is correct
Correct because NHRP authentication must match between hub and spoke for registration to succeed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The spoke's tunnel interface is in a different VRF than the hub's.
Why it's wrong here
Incorrect because VRF mismatch would prevent ping, but ping succeeds.
✗
The hub's tunnel interface has 'no ip nhrp server-only' configured.
Why it's wrong here
Incorrect because 'ip nhrp server-only' is not a standard command; the hub is the NHS by default.
✗
The spoke's NHRP map is incorrect; it should map the hub's tunnel IP to the hub's tunnel IP.
Why it's wrong here
Incorrect because the NHRP map maps the tunnel IP to the physical IP, which is correct.
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.
Trap categories for this question
Command / output trap
Incorrect because 'ip nhrp server-only' is not a standard command; the hub is the NHS by default.
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 hub router has 'ip nhrp authentication DMVPN' configured, but the spoke does not. — NHRP registration requires the spoke to send a Registration Request to the hub. If the hub does not respond, the spoke will not have NHRP entries. A common cause is that the hub's NHRP authentication is configured with a password, but the spoke's NHRP authentication is missing or mismatched.
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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