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300-410 Practice Question: An engineer configures OSPF on a DMVPN Phase 1…
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?
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.
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.
- ✗
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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