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300-410 Practice Question: In a DMVPN Phase 2 network with OSPF as the IGP,…
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.
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.
Why this is correct
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.
- ✗
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).
Why it's wrong here
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.
- ✗
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
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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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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