- A
The router with the larger MTU sends DBD packets that exceed the smaller MTU, causing them to be dropped silently.
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.
- B
The router with the smaller MTU cannot process OSPF hello packets from the larger MTU side.
Why wrong: Hello packets are small and not affected by MTU mismatch. The issue is with DBD packets.
- C
The adjacency is stuck because OSPF network type mismatch prevents DBD exchange.
Why wrong: Network type mismatch can cause issues, but the symptom of being stuck in EXSTART specifically points to MTU mismatch.
- D
The router with the larger MTU must have 'ip ospf mtu-ignore' configured to bypass the MTU check.
Why wrong: 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.
Quick Answer
The answer is that the router with the larger MTU sends DBD packets that exceed the smaller MTU, causing them to be silently dropped. This happens because OSPF includes the interface MTU value in the Database Description packet; when the receiving router’s interface has a smaller MTU, it discards the oversized DBD without any notification, halting the exchange process. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF neighbor state progression and the specific role of MTU in the EXSTART phase—a common trap is assuming the adjacency will fail earlier in the Down or Init state, but mismatched MTUs only block the DBD negotiation. Remember that OSPF does not fragment its own packets, so a silent drop occurs rather than an error message. A useful memory tip: “Big DBDs get dropped—EXSTART is where MTU stops the start.”
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
OSPF DBD packets carry the interface MTU in the OSPF header, and by default, a router will drop any OSPF packet whose size exceeds the configured MTU of the receiving interface. This behavior is defined in RFC 2328, Section 10.6, and the 'ip ospf mtu-ignore' command can be used to disable this check, allowing the adjacency to form even with mismatched MTUs. In real-world scenarios, this issue often arises when connecting to service provider links or when using GRE tunnels with reduced MTU.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 24, 2026
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.
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