The answer is that the router lacks a default route pointing to the 6rd tunnel interface. While the tunnel is up and the router itself can reach IPv6 hosts within the 6rd domain, the absence of a static route to ::/0 via the tunnel means that traffic destined for the native IPv6 internet has no path to the border relay. The border relay is correctly configured, but without this default route, the router cannot forward non-6rd IPv6 packets out the tunnel for encapsulation and delivery to the native internet. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of 6rd tunnel routing logic—specifically that the tunnel provides connectivity only to the 6rd domain, not to all IPv6 destinations. A common trap is assuming the tunnel’s up status implies full internet reachability. Remember the memory tip: “6rd tunnel up does not mean IPv6 internet—add a ::/0 route to the tunnel for the full trip.”
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 a 6rd tunnel on a router. The tunnel comes up and the router can reach IPv6 hosts on the 6rd domain, but IPv6 hosts behind the router cannot reach the internet (native IPv6). The 6rd border relay is configured correctly. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The 6rd router does not have a default route pointing to the tunnel interface; a static route to ::/0 via the tunnel is required.
The 6rd tunnel is up and the router can reach IPv6 hosts on the 6rd domain, but native IPv6 hosts behind the router cannot reach the internet. This indicates that the tunnel encapsulation works, but return traffic from the native IPv6 internet cannot reach the hosts because the router lacks a default route pointing to the tunnel interface. A static route to ::/0 via the tunnel interface is required to forward all non-6rd IPv6 traffic to the border relay, which then encapsulates it for the native IPv6 internet.
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 6rd router does not have a default route pointing to the tunnel interface; a static route to ::/0 via the tunnel is required.
Why this is correct
6rd does not automatically install a default route. Without a default route via the tunnel, traffic to native IPv6 destinations is dropped.
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 6rd prefix length is incorrect; it must be /32 for the tunnel to work.
Why it's wrong here
6rd prefix length can vary; it is not fixed to /32.
✗
The border relay is not configured with the same 6rd prefix.
Why it's wrong here
If the border relay were misconfigured, the tunnel would not come up.
✗
The router's IPv4 address is not in the 6rd delegated prefix range.
Why it's wrong here
The 6rd prefix is derived from the IPv4 address; if it were out of range, the tunnel would not form.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that a tunnel being up and reachable within the 6rd domain does not imply a default route exists for native IPv6 internet access, leading candidates to overlook the missing static route.
Detailed technical explanation
How to think about this question
6rd (RFC 5969) uses a stateless IPv6-in-IPv4 tunnel where the border relay provides connectivity to the native IPv6 internet. The 6rd router must have a default route (::/0) pointing to the tunnel interface so that any IPv6 destination not matching the local 6rd prefix is forwarded to the border relay. Without this route, the router will drop packets destined to non-6rd IPv6 addresses, even though the tunnel is operational for the 6rd domain.
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 6rd router does not have a default route pointing to the tunnel interface; a static route to ::/0 via the tunnel is required. — The 6rd tunnel is up and the router can reach IPv6 hosts on the 6rd domain, but native IPv6 hosts behind the router cannot reach the internet. This indicates that the tunnel encapsulation works, but return traffic from the native IPv6 internet cannot reach the hosts because the router lacks a default route pointing to the tunnel interface. A static route to ::/0 via the tunnel interface is required to forward all non-6rd IPv6 traffic to the border relay, which then encapsulates it for the native IPv6 internet.
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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