The answer is the reduced MTU on the tunnel interfaces is caused by GRE encapsulation adding 24 bytes of overhead. This is because Generic Routing Encapsulation (GRE) prepends a 4-byte outer IP header and a 4-byte GRE header to the original packet, which together with the inner IP header totals 24 bytes, reducing the maximum payload size from 1500 bytes on the physical GigabitEthernet interface to 1476 bytes on the tunnel. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how tunnel interfaces automatically adjust their MTU to account for encapsulation overhead, and a common trap is confusing this with IPsec overhead (which adds more bytes) or assuming a manual MTU command was used. Remember the memory tip: "GRE takes 24—four outer, four GRE, and the inner IP makes three fours."
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 runs the following command on Router R1:
R1# show ipv6 mtu
IPv6 MTU per interface:
Interface MTU
Tunnel0 1476
Tunnel1 1476
GigabitEthernet0/0 1500
Based on this output, what is the most likely reason for the reduced MTU on the tunnel interfaces?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The tunnels are using GRE encapsulation, which adds 24 bytes of overhead.
The output shows an MTU of 1476 on the tunnel interfaces, which is exactly 24 bytes less than the 1500-byte MTU of the physical GigabitEthernet0/0 interface. GRE encapsulation adds a 4-byte outer IP header and a 4-byte GRE header (total 24 bytes with the inner IP header), reducing the payload MTU. This is the default behavior when a tunnel is configured without a manual MTU override, making option B correct.
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 tunnels are configured with a manual MTU.
Why it's wrong here
While possible, the default behavior is to reduce MTU for encapsulation.
✓
The tunnels are using GRE encapsulation, which adds 24 bytes of overhead.
Why this is correct
GRE adds 4 bytes to the 20-byte IPv4 header, totaling 24 bytes overhead.
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 tunnels are using IPsec, which adds more overhead.
Why it's wrong here
IPsec overhead is typically larger and variable; 24 bytes is consistent with GRE.
✗
The tunnels are 6to4 tunnels, which do not reduce MTU.
Why it's wrong here
6to4 tunnels also reduce MTU due to IPv4 encapsulation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the exact overhead values for different tunnel types (GRE vs. IPsec vs. 6to4) to see if candidates know that GRE adds exactly 24 bytes, while IPsec adds more and 6to4 adds only 20 bytes.
Detailed technical explanation
How to think about this question
GRE encapsulation adds a 4-byte GRE header plus a 20-byte outer IPv4 header (total 24 bytes) to the original packet, so the tunnel interface MTU is automatically calculated as the outgoing interface MTU minus 24 bytes. This default behavior can be overridden with the 'ip mtu' command, but without it, the router dynamically adjusts the tunnel MTU. In real-world scenarios, this MTU reduction can cause fragmentation issues if not accounted for in path MTU discovery, especially with additional IPsec or other encapsulations.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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 tunnels are using GRE encapsulation, which adds 24 bytes of overhead. — The output shows an MTU of 1476 on the tunnel interfaces, which is exactly 24 bytes less than the 1500-byte MTU of the physical GigabitEthernet0/0 interface. GRE encapsulation adds a 4-byte outer IP header and a 4-byte GRE header (total 24 bytes with the inner IP header), reducing the payload MTU. This is the default behavior when a tunnel is configured without a manual MTU override, making option B correct.
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.
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