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300-410 Practice Question: An engineer configures OSPF on two directly…
An engineer configures OSPF on two directly connected routers with MTU 1500 on one interface and MTU 1400 on the other. The OSPF adjacency forms but remains 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 larger MTU sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART.
OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatch is such that the DBD packet from the larger MTU side is fragmented or dropped by the smaller MTU interface, the adjacency will stall in EXSTART. OSPF does not negotiate MTU; it simply compares the MTU value in the DBD packet. If the receiving interface has a smaller MTU, it will reject the DBD packet, causing the neighbor to stay in EXSTART.
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 sends DBD packets that exceed the smaller MTU, causing the receiver to drop them and remain in EXSTART.
Why this is correct
OSPF DBD packets include the MTU of the sending interface. If the receiving interface has a smaller MTU, it will ignore the DBD packet, preventing the adjacency from progressing.
- ✗
The router with the smaller MTU cannot send hello packets due to fragmentation, so the adjacency never forms.
Why it's wrong here
Hello packets are small and typically not affected by MTU. The issue is with DBD packets, not hellos.
- ✗
OSPF uses TCP, and the MTU mismatch causes TCP segmentation issues, leading to EXSTART.
Why it's wrong here
OSPF uses IP protocol 89, not TCP. MTU affects IP fragmentation, not TCP segmentation.
- ✗
The MTU mismatch causes a routing loop, preventing the exchange of LSAs.
Why it's wrong here
MTU mismatch does not cause routing loops; it directly affects packet delivery at Layer 2/Layer 3.
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.