Question 1,569 of 2,152

DMVPN →hardMultiple ChoiceObjective-mapped

300-410 DMVPN Practice Question

This 300-410 practice question tests your understanding of dmvpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 experienced network engineer configures a DMVPN Phase 2 network with OSPF as the routing protocol. On the hub router, the tunnel interface is configured with 'ip ospf network broadcast' and the spokes with 'ip ospf network point-to-multipoint'. Unexpectedly, the hub OSPF neighbor state with each spoke remains stuck in EXSTART/EXCHANGE. Which is the most likely explanation?

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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

A
The OSPF MTU mismatch between hub and spokes causes the adjacency to stall during the DD exchange phase.
Why wrong: MTU mismatch typically causes a stuck in EXSTART state, but here the network type mismatch is the primary issue; MTU mismatch would show a 'mtu mismatch' message in debug output.
B
The OSPF network type mismatch between hub (broadcast) and spokes (point-to-multipoint) prevents proper Hello and DD packet exchange, causing the stuck state.
Mismatched OSPF network types lead to different Hello packet formats (multicast vs unicast) and DR election behavior, preventing the adjacency from progressing beyond EXSTART/EXCHANGE.
C
The NHRP registration process is incomplete, causing OSPF to fail to establish neighbor relationships over the DMVPN tunnel.
Why wrong: NHRP registration is required for spoke-to-spoke communication but does not directly affect OSPF adjacency establishment; OSPF can form neighbors even without NHRP registration if the tunnel is up.
D
The hub router's OSPF priority is set to 0, preventing it from becoming the DR, which disrupts the adjacency formation.
Why wrong: Setting priority to 0 prevents the router from becoming DR, but a broadcast network still requires a DR; the spokes as point-to-multipoint do not participate in DR election, causing the mismatch.

Full breakdown with real-world context →

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 OSPF network type mismatch between hub (broadcast) and spokes (point-to-multipoint) prevents proper Hello and DD packet exchange, causing the stuck state.

OSPF requires matching network types to form a full adjacency. A broadcast network type expects a DR/BDR election and uses multicast Hellos, while point-to-multipoint uses unicast Hellos. The mismatch causes the hub to send Database Descriptor packets (DD) expecting a response from a DR, but the spoke, configured as point-to-multipoint, does not participate in DR election and responds differently, leading to a stuck state in EXSTART/EXCHANGE. The fix is to use 'ip ospf network broadcast' on all routers or use 'ip ospf network point-to-multipoint' consistently.

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 OSPF MTU mismatch between hub and spokes causes the adjacency to stall during the DD exchange phase.
Why it's wrong here
MTU mismatch typically causes a stuck in EXSTART state, but here the network type mismatch is the primary issue; MTU mismatch would show a 'mtu mismatch' message in debug output.
✓
The OSPF network type mismatch between hub (broadcast) and spokes (point-to-multipoint) prevents proper Hello and DD packet exchange, causing the stuck state.
Why this is correct
Mismatched OSPF network types lead to different Hello packet formats (multicast vs unicast) and DR election behavior, preventing the adjacency from progressing beyond EXSTART/EXCHANGE.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The NHRP registration process is incomplete, causing OSPF to fail to establish neighbor relationships over the DMVPN tunnel.
Why it's wrong here
NHRP registration is required for spoke-to-spoke communication but does not directly affect OSPF adjacency establishment; OSPF can form neighbors even without NHRP registration if the tunnel is up.
✗
The hub router's OSPF priority is set to 0, preventing it from becoming the DR, which disrupts the adjacency formation.
Why it's wrong here
Setting priority to 0 prevents the router from becoming DR, but a broadcast network still requires a DR; the spokes as point-to-multipoint do not participate in DR election, causing the mismatch.

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
MTU mismatch typically causes a stuck in EXSTART state, but here the network type mismatch is the primary issue; MTU mismatch would show a 'mtu mismatch' message in debug output.

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.

What to study next

Got this wrong? Here's your next step.

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.

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FAQ

Questions learners often ask

What does this 300-410 question test?

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 OSPF network type mismatch between hub (broadcast) and spokes (point-to-multipoint) prevents proper Hello and DD packet exchange, causing the stuck state. — OSPF requires matching network types to form a full adjacency. A broadcast network type expects a DR/BDR election and uses multicast Hellos, while point-to-multipoint uses unicast Hellos. The mismatch causes the hub to send Database Descriptor packets (DD) expecting a response from a DR, but the spoke, configured as point-to-multipoint, does not participate in DR election and responds differently, leading to a stuck state in EXSTART/EXCHANGE. The fix is to use 'ip ospf network broadcast' on all routers or use 'ip ospf network point-to-multipoint' consistently.

What should I do if I get this 300-410 question wrong?

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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