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300-410 Practice Question: Is troubleshooting a manual IPv6-in-IPv4 tunnel…
A network engineer is troubleshooting a manual IPv6-in-IPv4 tunnel between two Cisco routers. The tunnel is up, and both routers can ping each other's tunnel IPv6 addresses. However, traffic from a host behind Router A to a host behind Router B fails. The engineer notices that Router A has a route to the remote IPv6 prefix via the tunnel, but Router B does not have a route to the local IPv6 prefix. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between tunnel reachability (Layer 3 connectivity between tunnel endpoints) and prefix reachability (routing of actual user networks), leading candidates to overlook the missing static route on the return path.
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
✓
Router B is missing a static route pointing the local IPv6 prefix to the tunnel interface.
The tunnel is up and both routers can ping each other's tunnel IPv6 addresses, confirming that the tunnel itself is operational. However, traffic from a host behind Router A to a host behind Router B fails because Router B lacks a route back to the local IPv6 prefix (the network behind Router A). For bidirectional communication, both routers must have a route to the remote IPv6 prefix pointing to the tunnel interface. Since Router B is missing this static route, it cannot forward return traffic into the tunnel, causing the failure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Router B is missing a static route pointing the local IPv6 prefix to the tunnel interface.
Why this is correct
Correct because without a return route, Router B cannot forward packets destined to the local prefix, breaking bidirectional communication.
- ✗
The tunnel mode is set to 'ipv6ip 6to4' instead of 'ipv6ip'.
Why it's wrong here
Incorrect because 'tunnel mode ipv6ip' is correct for manual tunnels; 6to4 mode would change the encapsulation and addressing.
- ✗
The tunnel source on Router B is misconfigured with the wrong IPv4 address.
Why it's wrong here
Incorrect because if the tunnel is up and they can ping each other's tunnel IPv6 addresses, the tunnel source is correct.
- ✗
The IPv6 access-list on Router B is blocking incoming traffic from the local prefix.
Why it's wrong here
Incorrect because the issue is routing, not filtering; the engineer should check the routing table first.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.