hardMultiple Choice
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?
⚠ Common exam trap
300-410 often tests the specific symptom-to-cause mapping: EXSTART/EXCHANGE on a point-to-point link points to MTU mismatch, while other states (INIT, 2-WAY, or flapping) point to hello/dead timers, network type, or router ID issues.
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.
OSPF neighbors stuck in EXSTART/EXCHANGE state on a point-to-point link are most commonly caused by an MTU mismatch between the two interfaces. During the database description (DBD) exchange, OSPF includes the MTU in the DBD packets; if the MTU values do not match, the routers will not proceed to the EXCHANGE state and will remain in EXSTART. The fix is to align the MTU on both interfaces or use the 'ip ospf mtu-ignore' command.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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