hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An experienced network engineer configures a…
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?
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.
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.
- ✗
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.
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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Written by Johnson Ajibi, MSc IT Security
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