mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPv6 over IPv4 tunnel that…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
Go deeper
Related to this question
About these practice questions
One of 1,966 original 300-410 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.