Question 1,566 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 engineer configures OSPF on a DMVPN Phase 1 network with a single hub and multiple spokes. The hub is configured with 'ip ospf network broadcast' and the spokes with 'ip ospf network point-to-multipoint'. The hub's OSPF priority is set to 255, and all spokes have priority 0. Unexpectedly, the hub does not become the DR, and no OSPF adjacency is formed. 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 network type mismatch causes the hub to send multicast Hellos, but the spokes expect unicast Hellos, so no adjacency forms.
Broadcast network uses multicast 224.0.0.5, while point-to-multipoint uses unicast; without matching, Hellos are not received.
B
The hub's OSPF priority of 255 ensures it becomes the DR, but the spokes with priority 0 cannot become BDR, causing the election to fail.
Why wrong: Priority 0 means the router cannot become DR or BDR, but it can still form adjacency with the DR; the issue is network type mismatch.
C
The spokes are configured with 'ip ospf network point-to-multipoint' but the hub is broadcast; the hub will still form adjacencies with the spokes if the MTU matches.
Why wrong: MTU matching does not overcome network type mismatch; Hellos are not exchanged.
D
The OSPF process on the hub has a lower Router ID than the spokes, causing the spokes to become DR instead of the hub.
Why wrong: Router ID is used in DR election only if priorities are equal; here priority is set, but the network type mismatch prevents any election.

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 causes the hub to send multicast Hellos, but the spokes expect unicast Hellos, so no adjacency forms.

In OSPF, the DR election is based on priority and Router ID. However, on a broadcast network, all routers must have the same network type to participate in the election. With a mix of broadcast and point-to-multipoint, the point-to-multipoint routers do not participate in the DR election, and the broadcast router may still attempt to elect a DR. But if the hub is the only router with broadcast network type, it will become the DR (since no other routers participate). However, the adjacency may still fail because the point-to-multipoint routers do not respond to multicast Hellos from the hub. The corner case is that the hub's OSPF interface is configured as broadcast, but the spokes are point-to-multipoint, which means the spokes send unicast Hellos and expect unicast Hellos in return. The hub sends multicast Hellos, which the spokes ignore, and vice versa, leading to no adjacency.

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 network type mismatch causes the hub to send multicast Hellos, but the spokes expect unicast Hellos, so no adjacency forms.
Why this is correct
Broadcast network uses multicast 224.0.0.5, while point-to-multipoint uses unicast; without matching, Hellos are not received.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The hub's OSPF priority of 255 ensures it becomes the DR, but the spokes with priority 0 cannot become BDR, causing the election to fail.
Why it's wrong here
Priority 0 means the router cannot become DR or BDR, but it can still form adjacency with the DR; the issue is network type mismatch.
✗
The spokes are configured with 'ip ospf network point-to-multipoint' but the hub is broadcast; the hub will still form adjacencies with the spokes if the MTU matches.
Why it's wrong here
MTU matching does not overcome network type mismatch; Hellos are not exchanged.
✗
The OSPF process on the hub has a lower Router ID than the spokes, causing the spokes to become DR instead of the hub.
Why it's wrong here
Router ID is used in DR election only if priorities are equal; here priority is set, but the network type mismatch prevents any election.

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.

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 causes the hub to send multicast Hellos, but the spokes expect unicast Hellos, so no adjacency forms. — In OSPF, the DR election is based on priority and Router ID. However, on a broadcast network, all routers must have the same network type to participate in the election. With a mix of broadcast and point-to-multipoint, the point-to-multipoint routers do not participate in the DR election, and the broadcast router may still attempt to elect a DR. But if the hub is the only router with broadcast network type, it will become the DR (since no other routers participate). However, the adjacency may still fail because the point-to-multipoint routers do not respond to multicast Hellos from the hub. The corner case is that the hub's OSPF interface is configured as broadcast, but the spokes are point-to-multipoint, which means the spokes send unicast Hellos and expect unicast Hellos in return. The hub sends multicast Hellos, which the spokes ignore, and vice versa, leading to no adjacency.

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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