Question 1,071 of 2,152

IPv6 Tunneling Techniques →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the tunnel source interface is configured with a private IPv4 address, which causes the 6to4 prefix to be invalid. This is because a 6to4 tunnel automatically derives its IPv6 prefix from the tunnel source’s IPv4 address using the format 2002:IPv4-address::/48; when that source is a private address like 10.0.0.1, the resulting prefix 2002:0a00:0001::/48 is non-routable over the public Internet, as private addresses are not globally unique. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of 6to4 addressing rules and the common trap that a tunnel can show as up/up even when the underlying IPv4 source is invalid for routing. A frequent mistake is assuming interface status guarantees connectivity, but the real issue is the lack of a valid, globally routable 6to4 prefix. Remember the memory tip: “Private source, broken course” — if the tunnel source is private, the 2002::/16 prefix will never route.

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 connectivity issue between two sites connected via a 6to4 tunnel. The tunnel is configured on both routers and shows as up/up, but the engineer cannot ping the IPv6 address of the remote tunnel endpoint. The engineer checks the routing table and sees no route to the remote IPv6 prefix. What is the most likely cause of this problem?

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.

Question 1mediummultiple choice

Study the full IPv6 explanation →

A
The tunnel source interface is configured with a private IPv4 address, causing the 6to4 prefix to be invalid.
Correct because 6to4 requires a global IPv4 address to form a valid 2002::/16 prefix. A private address leads to an invalid 6to4 address, preventing proper routing.
B
The tunnel mode is incorrectly set to ipv6ip instead of 6to4.
Why wrong: Incorrect because 'tunnel mode ipv6ip' is used for manual tunnels, not 6to4. However, the tunnel would still show up/up but might not encapsulate correctly; the primary issue is the source address.
C
The tunnel destination is misconfigured with the remote router's IPv6 address instead of its IPv4 address.
Why wrong: Incorrect because 6to4 tunnels do not use a tunnel destination; they use automatic destination derivation from the 6to4 prefix. A misconfigured destination would not cause the tunnel to come up.
D
The IPv6 address on the tunnel interface is not in the 2002::/16 range.
Why wrong: Incorrect because the tunnel interface IPv6 address is not required to be in 2002::/16 for the tunnel to work; the tunnel endpoint addresses are derived from the source IPv4 address.

Full breakdown with real-world context →

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 tunnel source interface is configured with a private IPv4 address, causing the 6to4 prefix to be invalid.

For a 6to4 tunnel, the IPv6 address on the tunnel interface must be derived from the tunnel source's public IPv4 address using the 2002:IPv4-address::/48 prefix format. If the tunnel source interface has a private IPv4 address (e.g., 10.0.0.1), the resulting 6to4 prefix (2002:0a00:0001::/48) is non-routable over the public Internet because private addresses are not globally unique. This causes the remote router to have no route to the invalid prefix, breaking connectivity even though the tunnel interface is up/up.

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 source interface is configured with a private IPv4 address, causing the 6to4 prefix to be invalid.
Why this is correct
Correct because 6to4 requires a global IPv4 address to form a valid 2002::/16 prefix. A private address leads to an invalid 6to4 address, preventing proper routing.
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 mode is incorrectly set to ipv6ip instead of 6to4.
Why it's wrong here
Incorrect because 'tunnel mode ipv6ip' is used for manual tunnels, not 6to4. However, the tunnel would still show up/up but might not encapsulate correctly; the primary issue is the source address.
✗
The tunnel destination is misconfigured with the remote router's IPv6 address instead of its IPv4 address.
Why it's wrong here
Incorrect because 6to4 tunnels do not use a tunnel destination; they use automatic destination derivation from the 6to4 prefix. A misconfigured destination would not cause the tunnel to come up.
✗
The IPv6 address on the tunnel interface is not in the 2002::/16 range.
Why it's wrong here
Incorrect because the tunnel interface IPv6 address is not required to be in 2002::/16 for the tunnel to work; the tunnel endpoint addresses are derived from the source IPv4 address.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a 6to4 tunnel only requires the tunnel to be up/up, but the real issue is the routability of the derived 2002::/48 prefix when the source IPv4 address is private.

Trap categories for this question

Command / output trap
Incorrect because 'tunnel mode ipv6ip' is used for manual tunnels, not 6to4. However, the tunnel would still show up/up but might not encapsulate correctly; the primary issue is the source address.

Detailed technical explanation

How to think about this question

6to4 (RFC 3056) automatically derives a /48 IPv6 prefix from the tunnel source's public IPv4 address by encoding the IPv4 address in hexadecimal into the 2002::/16 prefix (e.g., 2002:c0a8:0101::/48 for 192.168.1.1). If the source is a private address (RFC 1918), the resulting 2002:0a00:0001::/48 prefix is not globally routable because the IPv4 address is not unique, causing the remote 6to4 router to drop packets or have no valid route. In real-world scenarios, this often occurs when engineers use loopback interfaces with private IPs or forget to use a public-facing interface as the tunnel source.

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 tunnel source interface is configured with a private IPv4 address, causing the 6to4 prefix to be invalid. — For a 6to4 tunnel, the IPv6 address on the tunnel interface must be derived from the tunnel source's public IPv4 address using the 2002:IPv4-address::/48 prefix format. If the tunnel source interface has a private IPv4 address (e.g., 10.0.0.1), the resulting 6to4 prefix (2002:0a00:0001::/48) is non-routable over the public Internet because private addresses are not globally unique. This causes the remote router to have no route to the invalid prefix, breaking connectivity even though the tunnel interface is up/up.

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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