The answer is an MTU mismatch on the tunnel interface causing IPv6 packet drops. In an L2TPv3 tunnel carrying IPv6 over IPv4, the tunnel interface MTU defaults to 1500 bytes, but the actual path MTU between the tunnel endpoints is lower due to the added L2TPv3, UDP, and IP headers. Since IPv6 routers do not fragment packets, they rely on ICMPv6 Packet Too Big messages to signal the sender to reduce the packet size; if these messages are blocked or not processed, packets exceeding the effective path MTU are silently dropped. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding of how overlay tunnels interact with IPv6’s no-fragmentation rule and the importance of adjusting the tunnel MTU or enabling path MTU discovery. A quick memory tip: for IPv6 over L2TPv3, always subtract the header overhead from the default MTU—think “L2TPv3 adds 40+ bytes, so set tunnel MTU to 1460 or lower.”
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 using IPv6 over IPv4 L2TPv3 tunnels is experiencing packet drops. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 tunnel mode l2tpv3 ipv6. Router R2 shows: R2# show l2tp tunnel Tunnel id 1, 1 session Session id 1, state established. R2# show ipv6 interface Tunnel0 Tunnel0 is up, line protocol is up. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The MTU on the tunnel interface is too high, causing fragmentation of IPv6 packets.
The correct answer is A. In an L2TPv3 tunnel carrying IPv6 traffic, the tunnel interface MTU defaults to 1500 bytes, but the actual path MTU between the tunnel endpoints (192.0.2.1 and 198.51.100.1) is likely lower due to the added L2TPv3, UDP, and IP headers. This causes IPv6 packets larger than the effective path MTU to be dropped because IPv6 routers do not fragment packets; they rely on ICMPv6 Packet Too Big messages, which may be blocked or not processed, leading to packet loss.
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 MTU on the tunnel interface is too high, causing fragmentation of IPv6 packets.
Why this is correct
L2TPv3 adds overhead, and if the tunnel MTU is not adjusted, packets may be fragmented or dropped.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel mode should be l2tpv3 ipv4 instead of ipv6.
Why it's wrong here
The mode is correct for transporting IPv6 over L2TPv3.
✗
The session ID on R1 does not match R2's session ID.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that L2TPv3 tunnels automatically handle MTU issues, when in fact the default MTU on the tunnel interface must be manually adjusted to prevent IPv6 packet drops due to encapsulation overhead.
Detailed technical explanation
How to think about this question
L2TPv3 encapsulates Layer 2 frames (including IPv6 packets) inside UDP/IP headers, adding 36–40 bytes of overhead (L2TPv3 session header + UDP + IP). The tunnel interface MTU must be reduced to account for this overhead, typically to 1460 bytes or less, to avoid fragmentation or drops. In real-world scenarios, path MTU discovery (PMTUD) failures due to ICMPv6 filtering are a common cause of silent packet drops in IPv6-over-L2TPv3 deployments.
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.
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 MTU on the tunnel interface is too high, causing fragmentation of IPv6 packets. — The correct answer is A. In an L2TPv3 tunnel carrying IPv6 traffic, the tunnel interface MTU defaults to 1500 bytes, but the actual path MTU between the tunnel endpoints (192.0.2.1 and 198.51.100.1) is likely lower due to the added L2TPv3, UDP, and IP headers. This causes IPv6 packets larger than the effective path MTU to be dropped because IPv6 routers do not fragment packets; they rely on ICMPv6 Packet Too Big messages, which may be blocked or not processed, leading to packet loss.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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