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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
Manual tunnels are stateless, so each router needs its own static route to the remote IPv6 prefix via the tunnel interface. Router B lacks that route, so return traffic cannot be encapsulated, satisfying the stem's asymmetric routing symptom.
- ✗
The tunnel mode is set to 'ipv6ip 6to4' instead of 'ipv6ip'.
Why it's wrong here
6to4 is an automatic tunnelling mode that derives the IPv6 prefix from the embedded IPv4 address, so it cannot carry the manually configured /64 prefix the stem describes; the tunnel would not come up or pass pings. It is tempting because 6to4 also encapsulates IPv6 in IPv4, and would be correct for automatic site-to-site tunnelling between public IPv4 addresses.
- ✗
The tunnel source on Router B is misconfigured with the wrong IPv4 address.
Why it's wrong here
A wrong tunnel source IPv4 address on Router B would prevent the tunnel interface from reaching up state or passing pings, yet the stem confirms both routers successfully ping each other's tunnel IPv6 addresses. It is tempting because mismatched tunnel sources are a frequent misconfiguration, and would be correct if the tunnel were down.
- ✗
The IPv6 access-list on Router B is blocking incoming traffic from the local prefix.
Why it's wrong here
An IPv6 access-list on Router B filtering inbound traffic would not remove Router B's own IPv6 routing table entry for the local prefix; the stem states that route is absent. It is tempting because ACLs commonly block tunnel traffic, and would be correct if the route existed but packets were being dropped.
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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.