The answer is that R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface. This causes manual tunnel one-way communication due to missing static route, because while R1 can send IPv6 traffic into the tunnel toward R2, R2 has no way to route return traffic back to R1’s tunnel subnet. Even with the tunnel interface up/up and full IPv4 reachability between endpoints, IPv6 routing is broken in one direction without a reciprocal static route on the far end. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that manual IPv6-in-IPv4 tunnels are point-to-point links requiring explicit static routes on both sides for the remote tunnel networks—a common trap where candidates assume tunnel up/up guarantees two-way traffic. Remember the memory tip: “Tunnel up is not route up; both sides need a static to complete the loop.”
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 large enterprise network is experiencing intermittent IPv6 connectivity between two remote sites connected via an IPv6-in-IPv4 manual tunnel. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 ipv6 route 2001:DB8:2::/64 Tunnel0. Router R2 shows: R2# show ipv6 route 2001:DB8:1::/64 % Route not found. The tunnel interface is up/up on both routers, and IPv4 reachability between tunnel endpoints is verified. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface.
The issue is that R2 has no route to the 2001:DB8:1::/64 network, which is the tunnel subnet on R1's side. Even though the tunnel is up/up and IPv4 reachability exists, R2 cannot return traffic to R1's tunnel endpoint address because it lacks a static route pointing to its own Tunnel0 interface. This is a common misconfiguration in point-to-point IPv6-in-IPv4 manual tunnels where both sides must have reciprocal static routes for the remote tunnel networks.
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 destination on R1 is incorrect and should be 198.51.100.2.
Why it's wrong here
The tunnel destination is the IPv4 address of R2's tunnel source, which is correct as given.
✓
R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface.
Why this is correct
Without a return route, R2 cannot forward packets back to R1's IPv6 network.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel mode must be changed to ipv6ip for IPv6-in-IPv4 tunneling.
Why it's wrong here
The default tunnel mode for manual tunnels is ipv6ip, which is correct.
✗
IPv6 unicast-routing is disabled on R2.
Why it's wrong here
While necessary, the show command indicates the route is missing, not that routing is disabled entirely.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that if the tunnel is up/up and IPv4 reachability is verified, IPv6 routing will automatically work, but they intentionally omit the reciprocal static route to trap candidates who overlook the need for explicit routing on both sides.
Trap categories for this question
Command / output trap
While necessary, the show command indicates the route is missing, not that routing is disabled entirely.
Detailed technical explanation
How to think about this question
In a manual IPv6-in-IPv4 tunnel (RFC 4213), each router encapsulates IPv6 packets in IPv4 headers using the configured tunnel source and destination. The tunnel interface is a point-to-point link, so each side must have a static route pointing to the remote IPv6 prefix via its own tunnel interface. Without this route, the router has no path to the remote tunnel network, causing asymmetric routing and intermittent connectivity. A common real-world scenario is when only one side configures the static route, leading to one-way traffic.
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: R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface. — The issue is that R2 has no route to the 2001:DB8:1::/64 network, which is the tunnel subnet on R1's side. Even though the tunnel is up/up and IPv4 reachability exists, R2 cannot return traffic to R1's tunnel endpoint address because it lacks a static route pointing to its own Tunnel0 interface. This is a common misconfiguration in point-to-point IPv6-in-IPv4 manual tunnels where both sides must have reciprocal static routes for the remote tunnel networks.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium
✓ A.Router B is missing a static route pointing the local IPv6 prefix to the tunnel interface.
B.The tunnel mode is set to 'ipv6ip 6to4' instead of 'ipv6ip'.
C.The tunnel source on Router B is misconfigured with the wrong IPv4 address.
D.The IPv6 access-list on Router B is blocking incoming traffic from the local prefix.
Why A: 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.
Last reviewed: Jun 24, 2026
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