- A
The IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
Correct because the tunnel MTU is typically 1500, but the IPv4 path may have an MTU of 1500 or less, leading to fragmentation when the 20-byte IP header is added. This causes errors and flapping.
- B
The tunnel keepalive is set too low, causing the tunnel to go down when a keepalive is lost.
Why wrong: Incorrect because keepalive loss would cause the tunnel to go down, not just flap with CRC errors.
- C
The tunnel source interface is a loopback that is not reachable from the remote router.
Why wrong: Incorrect because the tunnel is up, so the source is reachable.
- D
The tunnel is using GRE encapsulation, but the remote router is configured for ipv6ip.
Why wrong: Incorrect because a mode mismatch would prevent the tunnel from coming up at all.
Quick Answer
The answer is an MTU mismatch between the tunnel interface and the underlying IPv4 path, specifically when the path MTU is lower than the tunnel MTU plus 20 bytes. This mismatch forces fragmentation of the encapsulated packets, and the resulting fragmented packets are prone to loss and reassembly failures, which manifest as input errors and CRC errors on the tunnel interface. For the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how GRE tunnels interact with the IPv4 transport network—a classic troubleshooting trap is assuming the tunnel is stable just because it is up, when in reality fragmentation-induced packet loss causes the interface to flap as routing reconverges or the interface resets. Remember that GRE adds 20 bytes for the outer IP header plus 4 bytes for the GRE header, so the tunnel MTU must be set at least 24 bytes below the path MTU to avoid fragmentation. A simple memory tip: "Flapping from fragmentation means the path is too small for the tunnel's overhead call."
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. 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.
An engineer is troubleshooting an IPv6 manual tunnel between two routers that are not directly connected (the tunnel endpoints are separated by an IPv4 network). The tunnel is up, but the engineer notices that the tunnel interface flaps every few minutes. The show interfaces tunnel output shows input errors and CRC errors. What is the most likely cause?
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 IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
The tunnel interface shows input errors and CRC errors, which are classic signs of fragmentation and reassembly issues. When the IPv4 path between tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (the IPv4 header), packets exceeding the path MTU are fragmented. Fragmentation increases the risk of packet loss and CRC errors, causing the tunnel to flap as the interface resets or routing reconverges.
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 IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
Why this is correct
Correct because the tunnel MTU is typically 1500, but the IPv4 path may have an MTU of 1500 or less, leading to fragmentation when the 20-byte IP header is added. This causes errors and flapping.
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 tunnel keepalive is set too low, causing the tunnel to go down when a keepalive is lost.
Why it's wrong here
Incorrect because keepalive loss would cause the tunnel to go down, not just flap with CRC errors.
- ✗
The tunnel source interface is a loopback that is not reachable from the remote router.
Why it's wrong here
Incorrect because the tunnel is up, so the source is reachable.
- ✗
The tunnel is using GRE encapsulation, but the remote router is configured for ipv6ip.
Why it's wrong here
Incorrect because a mode mismatch would prevent the tunnel from coming up at all.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that tunnel flapping with CRC errors points to MTU/fragmentation issues, not reachability or encapsulation mismatches, which would prevent the tunnel from coming up at all.
Detailed technical explanation
How to think about this question
Manual IPv6 tunnels encapsulate IPv6 packets in IPv4 packets with a protocol type of 41. The tunnel interface MTU defaults to 1480 bytes (1500 minus 20 for IPv4 header), but if the underlying IPv4 path has a lower MTU (e.g., 1400), the 1480-byte IPv6 packet plus the 20-byte IPv4 header exceeds the path MTU, forcing fragmentation. Fragmented packets that are lost or corrupted cause CRC errors and input errors, leading to interface flapping as the router tries to recover.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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IPv6 Tunneling Techniques — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss. — The tunnel interface shows input errors and CRC errors, which are classic signs of fragmentation and reassembly issues. When the IPv4 path between tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (the IPv4 header), packets exceeding the path MTU are fragmented. Fragmentation increases the risk of packet loss and CRC errors, causing the tunnel to flap as the interface resets or routing reconverges.
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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