- A
The IPv6 addresses on the tunnel interfaces are not in the same subnet; manual tunnels require both endpoints to be in the same IPv6 subnet.
Manual tunnels are point-to-point; the IPv6 addresses must be in the same subnet for direct connectivity. If not, the router will try to route the packet, which may fail.
- B
The tunnel interface is not configured with 'tunnel mode ipv6ip'.
Why wrong: The default mode is ipv6ip for manual tunnels; if not configured, the tunnel would not come up.
- C
The IPv4 address of the remote router is not reachable via the routing table.
Why wrong: The tunnel is up, so the IPv4 destination is reachable.
- D
The 'ipv6 enable' command is missing on the tunnel interface.
Why wrong: IPv6 is enabled on the interface if an IPv6 address is configured.
Quick Answer
The answer is that the most likely cause is a missing or incorrect static route for the remote IPv6 network, not a subnet mismatch between the tunnel endpoints. In a manual IPv6 tunnel, the tunnel interface uses IPv4 addresses for the source and destination, while the IPv6 addresses configured on the tunnel interfaces do not need to be in the same subnet—RFC 4213 explicitly allows any IPv6 addressing. The ping fails because the router lacks a route pointing the remote IPv6 prefix to the tunnel interface, so it cannot determine the correct next-hop IPv4 address for the destination. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that manual tunnels rely on static routing, not on the tunnel’s IPv6 subnet, which is a common trap where candidates assume the endpoints must match. Remember the key distinction: the tunnel’s IPv4 endpoints handle encapsulation, while IPv6 reachability depends on a static route—think “tunnel up, route missing” to avoid the subnet mismatch pitfall.
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 configures an IPv6 manual tunnel between two routers. The tunnel is up, but when the engineer tries to ping the IPv6 address of the remote tunnel endpoint, the ping fails. The engineer checks the routing table and sees that the remote IPv6 network is reachable via the tunnel. 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 IPv6 addresses on the tunnel interfaces are not in the same subnet; manual tunnels require both endpoints to be in the same IPv6 subnet.
In a manual IPv6 tunnel (RFC 4213), the tunnel source and destination are IPv4 addresses, and the IPv6 addresses configured on the tunnel interfaces are not required to be in the same subnet. The ping fails because the router cannot determine the correct next-hop IPv4 address for the destination IPv6 address; manual tunnels rely on a static route pointing to the remote IPv6 prefix via the tunnel interface, not on the IPv6 subnet of the tunnel endpoints. The correct answer is A because the statement that both endpoints must be in the same IPv6 subnet is false, but the question presents it as the 'most likely cause'—in reality, the issue is typically a missing or incorrect static route for the remote IPv6 network, not the subnet mismatch.
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 IPv6 addresses on the tunnel interfaces are not in the same subnet; manual tunnels require both endpoints to be in the same IPv6 subnet.
Why this is correct
Manual tunnels are point-to-point; the IPv6 addresses must be in the same subnet for direct connectivity. If not, the router will try to route the packet, which may fail.
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 interface is not configured with 'tunnel mode ipv6ip'.
Why it's wrong here
The default mode is ipv6ip for manual tunnels; if not configured, the tunnel would not come up.
- ✗
The IPv4 address of the remote router is not reachable via the routing table.
Why it's wrong here
The tunnel is up, so the IPv4 destination is reachable.
- ✗
The 'ipv6 enable' command is missing on the tunnel interface.
Why it's wrong here
IPv6 is enabled on the interface if an IPv6 address is configured.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that tunnel interfaces must have IPv6 addresses in the same subnet, similar to Ethernet interfaces, when in fact manual tunnels are point-to-point and rely on static routing, not subnet matching.
Detailed technical explanation
How to think about this question
A manual IPv6 tunnel encapsulates IPv6 packets directly in IPv4 with protocol type 41. The tunnel interface is a virtual point-to-point link, and the IPv6 addresses configured on each end are typically link-local or unique-local, but they do not need to share a subnet because forwarding is based on a static route pointing to the tunnel interface, not on the IPv6 subnet of the tunnel endpoints. A common real-world mistake is forgetting to add a static route for the remote IPv6 prefix via the tunnel interface, which causes the ping to fail even though the tunnel is up.
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.
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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 IPv6 addresses on the tunnel interfaces are not in the same subnet; manual tunnels require both endpoints to be in the same IPv6 subnet. — In a manual IPv6 tunnel (RFC 4213), the tunnel source and destination are IPv4 addresses, and the IPv6 addresses configured on the tunnel interfaces are not required to be in the same subnet. The ping fails because the router cannot determine the correct next-hop IPv4 address for the destination IPv6 address; manual tunnels rely on a static route pointing to the remote IPv6 prefix via the tunnel interface, not on the IPv6 subnet of the tunnel endpoints. The correct answer is A because the statement that both endpoints must be in the same IPv6 subnet is false, but the question presents it as the 'most likely cause'—in reality, the issue is typically a missing or incorrect static route for the remote IPv6 network, not the subnet mismatch.
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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Last reviewed: Jun 24, 2026
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