- A
The IPsec transform set uses AES-256, which requires more CPU and causes performance drops.
Why wrong: Incorrect because CPU load does not cause intermittent drops; fragmentation is the issue.
- B
The tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops.
Correct because the tunnel MTU of 1400 bytes does not account for IPsec overhead, leading to fragmentation and drops.
- C
The physical interface MTU is set to 1500, which is too high for DMVPN.
Why wrong: Incorrect because the physical interface MTU is standard; the tunnel MTU should be lower to accommodate IPsec overhead.
- D
The spokes have different IPsec transform sets configured.
Why wrong: Incorrect because mismatched transform sets would prevent IPsec negotiation, not cause intermittent drops.
Quick Answer
The answer is that the tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops. In a DMVPN Phase 2 network, IPsec adds significant overhead—with AES-256 and SHA-256, the ESP header, trailer, and authentication data typically consume 50–60 bytes. When the tunnel interface MTU is set to 1400 bytes on a physical interface with a 1500-byte MTU, the effective payload MTU becomes roughly 1340–1350 bytes. Any packet exceeding this reduced size must be fragmented, and if the DF bit is set or intermediate routers drop fragments, spoke-to-spoke traffic will intermittently drop. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of IPsec overhead calculations and the relationship between tunnel MTU and fragmentation in DMVPN Phase 2. A common trap is assuming the tunnel MTU should match the physical MTU; instead, you must subtract IPsec overhead. Memory tip: “MTU minus overhead equals payload—if you don’t subtract, packets get cut.”
300-410 DMVPN Practice Question
This 300-410 practice question tests your understanding of dmvpn. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A network engineer is troubleshooting a DMVPN phase 2 network where spoke-to-spoke tunnels are established, but traffic between spokes is intermittently dropped. The engineer captures packets and sees that IPsec packets are being fragmented. The tunnel interface MTU is set to 1400 bytes, and the physical interface MTU is 1500 bytes. The engineer also notices that the IPsec transform set uses ESP with AES-256 and SHA-256. What is the most likely cause of the intermittent drops?
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 tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops.
IPsec adds overhead (ESP header, trailer, and authentication data). With AES-256 and SHA-256, the overhead can be around 50-60 bytes. If the tunnel MTU is set to 1400, the effective payload MTU is lower. Fragmentation can cause drops if the DF bit is set or if intermediate routers drop fragments. The issue is that the tunnel MTU is too high for the IPsec overhead, causing fragmentation.
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 IPsec transform set uses AES-256, which requires more CPU and causes performance drops.
Why it's wrong here
Incorrect because CPU load does not cause intermittent drops; fragmentation is the issue.
- ✓
The tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops.
- ✗
The physical interface MTU is set to 1500, which is too high for DMVPN.
- ✗
The spokes have different IPsec transform sets configured.
Why it's wrong here
Incorrect because mismatched transform sets would prevent IPsec negotiation, not cause intermittent drops.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- 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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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FAQ
Questions learners often ask
What does this 300-410 question test?
DMVPN — This question tests DMVPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The tunnel MTU is set too high for the IPsec overhead, causing fragmentation and potential drops. — IPsec adds overhead (ESP header, trailer, and authentication data). With AES-256 and SHA-256, the overhead can be around 50-60 bytes. If the tunnel MTU is set to 1400, the effective payload MTU is lower. Fragmentation can cause drops if the DF bit is set or if intermediate routers drop fragments. The issue is that the tunnel MTU is too high for the IPsec overhead, causing fragmentation.
What should I do if I get this 300-410 question wrong?
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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.
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Last reviewed: Jun 18, 2026
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