The answer is that the NAT64 prefix is missing from the tunnel interface configuration. NAT64 relies on a defined prefix, typically the well-known 64:FF9B::/96 or a network-specific prefix, to map IPv4 addresses into the IPv6 address space; without the `nat64 prefix` command under the tunnel interface, the router cannot construct the necessary IPv6 representation of IPv4 destinations, so no translation mappings are created, and packets are dropped—as confirmed by the debug output showing “No mapping found for packet.” On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that NAT64 is not a stateless tunnel but a stateful translation mechanism requiring explicit prefix configuration, and a common trap is assuming that enabling NAT64 globally or on the interface alone is sufficient. Remember the mnemonic: “No prefix, no translation—packets face annihilation.”
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 enterprise is using IPv6-to-IPv4 translation tunneling (NAT64) but users report that they cannot reach IPv6-only servers. 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 nat64 enable. Router R2 shows: R2# show nat64 translations % No translations. R2# debug nat64 all NAT64: No mapping found for packet. 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 NAT64 prefix is not configured under the tunnel interface.
Option A is correct because NAT64 requires a NAT64 prefix to be explicitly configured under the tunnel interface using the 'nat64 prefix' command. Without this prefix, the router cannot construct the IPv6 representation of IPv4 destinations, so no translation mappings are created, and packets are dropped. The debug output confirms 'No mapping found for packet', which directly points to the missing prefix configuration.
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 NAT64 prefix is not configured under the tunnel interface.
Why this is correct
Without a NAT64 prefix, the router cannot create mappings for translation.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel mode should be ipv6ip for NAT64 to work.
Why it's wrong here
NAT64 works with GRE or other tunnel modes; ipv6ip is not required.
✗
The IPv4 address of the destination server is not reachable.
Why it's wrong here
The debug shows no mapping, not unreachability.
✗
NAT64 is not supported on tunnel interfaces.
Why it's wrong here
NAT64 is supported on tunnel interfaces with appropriate configuration.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between enabling NAT64 on an interface and actually configuring the required NAT64 prefix, leading candidates to assume that 'nat64 enable' alone is sufficient.
Trap categories for this question
Command / output trap
The debug shows no mapping, not unreachability.
Detailed technical explanation
How to think about this question
NAT64 relies on a configured prefix (typically 64:ff9b::/96 or a custom one) to map IPv4 addresses into the IPv6 address space. When a packet arrives at the tunnel interface, the router uses this prefix to create dynamic translations; without it, the 'nat64 enable' command on the interface has no effect. In real-world deployments, forgetting to configure the prefix is a common oversight that leads to silent packet drops, as the router cannot perform the address synthesis required for IPv6-to-IPv4 communication.
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 NAT64 prefix is not configured under the tunnel interface. — Option A is correct because NAT64 requires a NAT64 prefix to be explicitly configured under the tunnel interface using the 'nat64 prefix' command. Without this prefix, the router cannot construct the IPv6 representation of IPv4 destinations, so no translation mappings are created, and packets are dropped. The debug output confirms 'No mapping found for packet', which directly points to the missing prefix configuration.
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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