- 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.
Why wrong: Incorrect. Manual tunnels do not require IPv6 addresses to be in the same subnet; they are point-to-point links and can use any IPv6 addresses. The subnet mismatch is not the cause.
- B
The tunnel interface is not configured with 'tunnel mode ipv6ip'.
Why wrong: Incorrect. The tunnel mode 'ipv6ip' is required for manual tunnels. If missing, the tunnel would not be up. The scenario states the tunnel is up, so this command is configured.
- C
The IPv4 address of the remote router is not reachable via the routing table.
Correct. The IPv4 address of the remote router must be reachable for the encapsulation to work. If it is not reachable, the tunnel may be up but traffic fails.
- D
The 'ipv6 enable' command is missing on the tunnel interface.
Why wrong: Incorrect. The 'ipv6 enable' command is not required on tunnel interfaces; it is used on interfaces that need IPv6 without a specific address. The tunnel interface already has an IPv6 address configured.
Manual Tunnel Endpoint Unreachable
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. A key principle to apply: manual IPv6 Tunnel. 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.
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.
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 IPv4 address of the remote router is not reachable via the routing table.
In a manual IPv6 tunnel (RFC 4213), the tunnel interface creates a virtual point-to-point link, but the encapsulated packets still rely on IPv4 reachability to the remote tunnel endpoint. If the IPv4 address of the remote router is not reachable (due to missing routes or connectivity issues), the tunnel interface may appear up, but the encapsulated ping packets will not reach the destination, causing failure. Option C correctly identifies this as the most likely cause because the routing table shows the IPv6 network reachable via the tunnel, but the IPv4 path is broken.
Key principle: Manual IPv6 Tunnel
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 it's wrong here
Incorrect. Manual tunnels do not require IPv6 addresses to be in the same subnet; they are point-to-point links and can use any IPv6 addresses. The subnet mismatch is not the cause.
- ✗
The tunnel interface is not configured with 'tunnel mode ipv6ip'.
Why it's wrong here
Incorrect. The tunnel mode 'ipv6ip' is required for manual tunnels. If missing, the tunnel would not be up. The scenario states the tunnel is up, so this command is configured.
- ✓
The IPv4 address of the remote router is not reachable via the routing table.
Why this is correct
Correct. The IPv4 address of the remote router must be reachable for the encapsulation to work. If it is not reachable, the tunnel may be up but traffic fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Manual IPv6 Tunnel
- ✗
The 'ipv6 enable' command is missing on the tunnel interface.
Why it's wrong here
Incorrect. The 'ipv6 enable' command is not required on tunnel interfaces; it is used on interfaces that need IPv6 without a specific address. The tunnel interface already has an IPv6 address configured.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Common mistake: assuming that a tunnel being up means the encapsulated path works. Actually, the tunnel interface state only indicates IPv6 connectivity, not IPv4 reachability of the remote endpoint.
Trap categories for this question
Command / output trap
Incorrect. The tunnel mode 'ipv6ip' is required for manual tunnels. If missing, the tunnel would not be up. The scenario states the tunnel is up, so this command is configured.
Scenario analysis trap
Incorrect. The tunnel mode 'ipv6ip' is required for manual tunnels. If missing, the tunnel would not be up. The scenario states the tunnel is up, so this command is configured.
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
- Manual IPv6 Tunnel
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
Manual IPv6 Tunnel
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review manual IPv6 Tunnel, then practise related 300-410 questions on the same topic to reinforce the concept.
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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 — Manual IPv6 Tunnel.
What is the correct answer to this question?
The correct answer is: The IPv4 address of the remote router is not reachable via the routing table. — In a manual IPv6 tunnel (RFC 4213), the tunnel interface creates a virtual point-to-point link, but the encapsulated packets still rely on IPv4 reachability to the remote tunnel endpoint. If the IPv4 address of the remote router is not reachable (due to missing routes or connectivity issues), the tunnel interface may appear up, but the encapsulated ping packets will not reach the destination, causing failure. Option C correctly identifies this as the most likely cause because the routing table shows the IPv6 network reachable via the tunnel, but the IPv4 path is broken.
What should I do if I get this 300-410 question wrong?
Review manual IPv6 Tunnel, then practise related 300-410 questions on the same topic to reinforce the concept.
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?
Manual IPv6 Tunnel
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Last reviewed: Jun 24, 2026
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