- A
OSPF network type mismatch between R2 (broadcast) and R3 (point-to-multipoint) prevents adjacency formation because the multicast and unicast OSPF packets are not compatible.
Correct. OSPF network types must match on all routers in the same network segment for adjacencies to form. Broadcast uses multicast, point-to-multipoint uses unicast, causing communication failure.
- B
R3's OSPF priority is set to 0, preventing it from becoming DR/BDR.
Why wrong: Incorrect. Priority 0 prevents DR/BDR election, but adjacencies can still form with DR/BDR. The issue is network type mismatch.
- C
R2 has an ACL blocking OSPF multicast traffic (224.0.0.5/6).
Why wrong: Incorrect. While an ACL could block OSPF, the question states R2 can ping R3, indicating basic connectivity. The issue is specific to OSPF network type.
- D
The DMVPN tunnel is not in Phase 2 mode, preventing spoke-to-spoke adjacencies.
Why wrong: Incorrect. DMVPN Phase 2 supports spoke-to-spoke tunnels, but OSPF adjacencies require matching network types.
Quick Answer
The answer is an OSPF network type mismatch between R2 (broadcast) and R3 (point-to-multipoint) on the DMVPN tunnel interface. This mismatch prevents adjacency formation because broadcast network types rely on multicast addresses 224.0.0.5 and 224.0.0.6 for hello packets and require a DR/BDR election, while point-to-multipoint uses unicast hellos and has no DR/BDR process. When R2 sends multicast OSPF packets, R3, expecting unicast, will ignore them, and vice versa, causing the adjacency to fail even though Layer 3 reachability exists. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF network types must be consistent across all DMVPN spokes to maintain neighbor relationships, and it is a common trap where candidates assume ping success guarantees OSPF adjacency. A useful memory tip: “Multicast meets unicast? No adjacency will last.”
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.
In a DMVPN Phase 2 network with OSPF as the IGP, R1 (hub) and R2 (spoke) are configured with 'ip ospf network broadcast' on the tunnel interface. R3 (another spoke) has 'ip ospf network point-to-multipoint'. R2 can ping R3's tunnel IP, but OSPF adjacencies between R2 and R3 are not forming. What is the root cause?
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
OSPF network type mismatch between R2 (broadcast) and R3 (point-to-multipoint) prevents adjacency formation because the multicast and unicast OSPF packets are not compatible.
OSPF network type mismatch on the DMVPN tunnel prevents adjacency formation. Broadcast network type requires a DR/BDR election and uses multicast 224.0.0.5/6, while point-to-multipoint uses unicast and does not elect DR/BDR. The mismatch causes OSPF packets to be ignored or not processed correctly.
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.
- ✓
OSPF network type mismatch between R2 (broadcast) and R3 (point-to-multipoint) prevents adjacency formation because the multicast and unicast OSPF packets are not compatible.
- ✗
R3's OSPF priority is set to 0, preventing it from becoming DR/BDR.
Why it's wrong here
Incorrect. Priority 0 prevents DR/BDR election, but adjacencies can still form with DR/BDR. The issue is network type mismatch.
- ✗
R2 has an ACL blocking OSPF multicast traffic (224.0.0.5/6).
- ✗
The DMVPN tunnel is not in Phase 2 mode, preventing spoke-to-spoke adjacencies.
Why it's wrong here
Incorrect. DMVPN Phase 2 supports spoke-to-spoke tunnels, but OSPF adjacencies require matching network types.
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: OSPF network type mismatch between R2 (broadcast) and R3 (point-to-multipoint) prevents adjacency formation because the multicast and unicast OSPF packets are not compatible. — OSPF network type mismatch on the DMVPN tunnel prevents adjacency formation. Broadcast network type requires a DR/BDR election and uses multicast 224.0.0.5/6, while point-to-multipoint uses unicast and does not elect DR/BDR. The mismatch causes OSPF packets to be ignored or not processed correctly.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
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Same concept, more angles
3 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hard- A.The OSPF MTU mismatch between hub and spokes causes the adjacency to stall during the DD exchange phase.
- ✓ 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.
- C.The NHRP registration process is incomplete, causing OSPF to fail to establish neighbor relationships over the DMVPN tunnel.
- D.The hub router's OSPF priority is set to 0, preventing it from becoming the DR, which disrupts the adjacency formation.
Why B: 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.
Variation 2. 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?
hard- ✓ A.The OSPF network type mismatch causes the hub to send multicast Hellos, but the spokes expect unicast Hellos, so no adjacency forms.
- 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.
- 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.
- 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 A: 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.
Variation 3. A network engineer configures a DMVPN spoke with OSPF as the routing protocol: interface Tunnel0 ip address 10.0.0.2 255.255.255.0 ip nhrp network-id 100 ip nhrp nhs 10.0.0.1 tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ip nhrp map 10.0.0.1 192.168.1.1 ip nhrp map multicast 192.168.1.1 ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0 ! What is a common issue with OSPF in this DMVPN Phase 2 configuration?
medium- ✓ A.OSPF will elect a DR/BDR on the hub, which can cause suboptimal routing and adjacency issues.
- B.OSPF will not form adjacencies because of NHRP authentication.
- C.OSPF will use point-to-point network type by default.
- D.OSPF will automatically adjust to the DMVPN environment.
Why A: OSPF over DMVPN Phase 2 requires the hub to be configured as an OSPF point-to-multipoint network type to avoid issues with DR/BDR elections and to allow spoke-to-spoke adjacencies.
Last reviewed: Jun 18, 2026
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