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300-410 Practice Question: An engineer configures OSPF on two routers…
An engineer configures OSPF on two routers connected via a serial link. The MTU on one side is 1500 and on the other is 1400. The OSPF adjacency forms but stays stuck in EXSTART state. 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 router with the smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs.
OSPF uses the interface MTU to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets from the neighbor, causing the adjacency to remain in EXSTART state.
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 router with the larger MTU cannot process OSPF hello packets from the neighbor.
Why it's wrong here
Hello packets are small and not affected by MTU mismatch.
- ✓
The router with the smaller MTU rejects DBD packets that exceed its MTU, preventing the exchange of LSAs.
Why this is correct
DBD packets must fit within the interface MTU; a mismatch causes the adjacency to stall in EXSTART.
- ✗
The OSPF network type must be point-to-point for serial links; otherwise, the adjacency fails.
Why it's wrong here
Network type affects DR/BDR election but not MTU handling.
- ✗
The OSPF process ID must match on both routers for adjacency to form.
Why it's wrong here
Process ID is locally significant and does not need to match.
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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.