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300-410 Practice Question: An engineer configures OSPF on a link between two…
An engineer configures OSPF on a link between two routers with MTU 1500 on one side and MTU 1400 on the other. The adjacency forms but is stuck in EXSTART. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the specific state where the adjacency gets stuck (EXSTART) to distinguish between MTU mismatch and other OSPF issues, and the trap here is that candidates may incorrectly attribute the problem to hello packet failures or network type mismatches rather than the silent dropping of DBD packets due to MTU mismatch.
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 them to be dropped silently.
When OSPF routers have mismatched MTUs, the router with the larger MTU (1500) will send Database Description (DBD) packets that include the full MTU size in the interface MTU field. The router with the smaller MTU (1400) will reject these packets because they exceed its MTU, causing them to be silently dropped. This prevents the DBD exchange from completing, leaving the adjacency stuck 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 sends DBD packets that exceed the smaller MTU, causing them to be dropped silently.
Why this is correct
OSPF DBD packets are sized based on the outgoing interface MTU. If the packet is larger than the receiving interface MTU, it is dropped, preventing the exchange of LSAs.
- ✗
The router with the smaller MTU cannot process OSPF hello packets from the larger MTU side.
Why it's wrong here
Hello packets are small and not affected by MTU mismatch. The issue is with DBD packets.
- ✗
The adjacency is stuck because OSPF network type mismatch prevents DBD exchange.
Why it's wrong here
Network type mismatch can cause issues, but the symptom of being stuck in EXSTART specifically points to MTU mismatch.
- ✗
The router with the larger MTU must have 'ip ospf mtu-ignore' configured to bypass the MTU check.
Why it's wrong here
While 'ip ospf mtu-ignore' can work around the issue, it must be configured on the router with the smaller MTU to ignore the DBD packet size, not the larger one.
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.