hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures OSPF on two routers…
An engineer configures OSPF on two routers connected via a point-to-point link. The routers are stuck in EXSTART state. 'show ip ospf neighbor' shows neighbor state 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 MTU on the interfaces is mismatched.
In OSPF, the EXSTART state involves negotiating the master/slave relationship and the Database Description (DBD) packet size. If the MTU on the interfaces differs, the larger DBD packet from the router with the larger MTU will be dropped by the router with the smaller MTU, causing the adjacency to remain stuck in EXSTART. This is a common edge case that is often overlooked.
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 is mismatched (point-to-point vs broadcast).
Why it's wrong here
A network type mismatch typically causes the neighbor to get stuck in INIT or 2-WAY, not EXSTART.
- ✓
The MTU on the interfaces is mismatched.
Why this is correct
An MTU mismatch prevents DBD packets from being exchanged, causing the adjacency to remain in EXSTART.
- ✗
The OSPF router IDs are identical.
Why it's wrong here
Duplicate router IDs would prevent the adjacency from forming at all, typically resulting in a state of DOWN or INIT.
- ✗
The hello/dead intervals are mismatched.
Why it's wrong here
Mismatched hello/dead intervals would prevent the neighbor from reaching the 2-WAY state, not EXSTART.
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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