300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. 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.
Given this configuration on Router R6:
```
interface Tunnel0
ipv6 address 2001:DB8:8::1/64
tunnel source 2001:DB8:9::1
tunnel destination 2001:DB8:10::2
tunnel mode gre ipv6
tunnel path-mtu-discovery
```
What is the effect?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The tunnel will dynamically adjust the MTU based on the path MTU discovery.
The `tunnel path-mtu-discovery` command enables Path MTU Discovery (PMTUD) for the GRE tunnel, which dynamically determines the maximum MTU along the path to the tunnel destination and adjusts the tunnel interface's MTU accordingly. This prevents fragmentation issues by allowing the tunnel to use the smallest MTU in the path. Option B correctly identifies this behavior.
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 tunnel will not work because path-mtu-discovery is not supported with GRE.
The tunnel will dynamically adjust the MTU based on the path MTU discovery.
Why this is correct
The 'tunnel path-mtu-discovery' command enables PMTUD for the tunnel.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel mode should be 'ipv6ip' for IPv6 transport.
Why it's wrong here
GRE over IPv6 is correctly configured; ipv6ip is for IPv4 transport.
✗
The tunnel source and destination must be IPv4 addresses.
Why it's wrong here
For gre ipv6, source and destination are IPv6.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that path-mtu-discovery is unsupported with GRE tunnels, but it is actually supported and commonly used to avoid fragmentation in overlay networks.
Detailed technical explanation
How to think about this question
Path MTU Discovery for GRE tunnels works by setting the Don't Fragment (DF) bit in the outer IPv6 header (or using the IPv6 equivalent) and relying on ICMPv6 Packet Too Big messages from intermediate routers to learn the path MTU. The tunnel interface then dynamically adjusts its MTU to avoid fragmentation, which is critical for performance in networks with varying link MTUs, such as VPNs traversing the Internet. Note that PMTUD can fail if ICMP messages are blocked by firewalls, leading to black-hole connections.
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 tunnel will dynamically adjust the MTU based on the path MTU discovery. — The `tunnel path-mtu-discovery` command enables Path MTU Discovery (PMTUD) for the GRE tunnel, which dynamically determines the maximum MTU along the path to the tunnel destination and adjusts the tunnel interface's MTU accordingly. This prevents fragmentation issues by allowing the tunnel to use the smallest MTU in the path. Option B correctly identifies this behavior.
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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