- A
The remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
Correct because the tunnel source loopback address must be reachable from the remote router. If the default route does not include that specific prefix (e.g., due to routing policy or subnet mismatch), the tunnel cannot encapsulate packets.
- B
The tunnel destination is configured with the loopback address of the remote router, but the remote router's tunnel source is a different interface.
Why wrong: Incorrect because the tunnel destination should be the IPv4 address of the remote tunnel endpoint, which could be a loopback. The issue is reachability, not mismatch.
- C
The tunnel interface is missing the 'tunnel mode ipv6ip' command.
Why wrong: Incorrect because the tunnel is up, so the mode is likely correct.
- D
The IPv6 address on the tunnel interface is not in the same subnet as the remote tunnel IPv6 address.
Why wrong: Incorrect because the tunnel endpoints can be in different subnets; the tunnel is a point-to-point link.
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.
A network engineer is troubleshooting an IPv6 over IPv4 tunnel that is used to connect two remote sites. The tunnel is configured with a tunnel source that is a loopback interface. The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address. The engineer checks the routing table and sees a route to the remote loopback's IPv4 address via a default route. 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 remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address because the remote router lacks a route back to the loopback network used as the tunnel source. The remote router only has a default route, which may not cover the specific prefix of the local loopback, causing return traffic to be dropped. For IPv6 over IPv4 tunnels, the tunnel source and destination must be reachable via unicast routing; a missing or insufficient route (like a default that doesn't match) breaks bidirectional communication.
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 remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
Why this is correct
Correct because the tunnel source loopback address must be reachable from the remote router. If the default route does not include that specific prefix (e.g., due to routing policy or subnet mismatch), the tunnel cannot encapsulate packets.
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 destination is configured with the loopback address of the remote router, but the remote router's tunnel source is a different interface.
Why it's wrong here
Incorrect because the tunnel destination should be the IPv4 address of the remote tunnel endpoint, which could be a loopback. The issue is reachability, not mismatch.
- ✗
The tunnel interface is missing the 'tunnel mode ipv6ip' command.
Why it's wrong here
Incorrect because the tunnel is up, so the mode is likely correct.
- ✗
The IPv6 address on the tunnel interface is not in the same subnet as the remote tunnel IPv6 address.
Why it's wrong here
Incorrect because the tunnel endpoints can be in different subnets; the tunnel is a point-to-point link.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a tunnel being up guarantees end-to-end reachability, but the real issue is asymmetric routing caused by missing return routes for the tunnel source IPv4 address.
Detailed technical explanation
How to think about this question
In IPv6 over IPv4 manual tunnels (RFC 4213), the tunnel source and destination are IPv4 addresses, and the tunnel interface creates a point-to-point link. The remote router must have a route to the IPv4 address of the tunnel source (the local loopback) to send return packets; a default route may not suffice if it points to a different next-hop or is ambiguous. This is a common misconfiguration in hub-and-spoke topologies where the spoke only has a default route that does not include the hub's loopback prefix.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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 remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix. — The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address because the remote router lacks a route back to the loopback network used as the tunnel source. The remote router only has a default route, which may not cover the specific prefix of the local loopback, causing return traffic to be dropped. For IPv6 over IPv4 tunnels, the tunnel source and destination must be reachable via unicast routing; a missing or insufficient route (like a default that doesn't match) breaks bidirectional communication.
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
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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