- A
The uRPF strict mode check fails because the router does not have a route to the source IPv6 network pointing to the tunnel interface.
Correct because uRPF on a tunnel requires a route to the source via the tunnel interface; without it, the packet is dropped.
- B
The ACL is blocking the IPv6 traffic because the tunnel interface does not support ACLs.
Why wrong: Incorrect because ACLs can be applied to tunnel interfaces.
- C
The tunnel is not configured with the correct IPv4 source and destination.
Why wrong: Incorrect because the tunnel is up and traffic is being received, indicating the tunnel configuration is correct.
- D
The uRPF mode should be loose mode to allow traffic from any source.
Why wrong: Incorrect because loose mode would still require a route to the source, but the issue is the route pointing to the tunnel interface; loose mode might work if there is any route, but the scenario implies strict mode is the problem.
Quick Answer
The answer is that uRPF strict mode on the tunnel interface drops the IPv6 traffic because the router lacks a route to the source IPv6 network pointing back to that specific tunnel interface. When uRPF strict mode is enabled, the router performs a reverse path lookup on the source IPv6 address of incoming decapsulated packets; if the best return route in the FIB does not point to the same tunnel interface where the packet arrived, the packet is dropped. In this scenario, the IPv6 traffic originates from a network behind the tunnel, so the router likely has a route to that source via a different interface—such as a physical LAN interface—or no route at all, causing the uRPF check to fail. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how uRPF interacts with tunnel interfaces and IPv6 routing, often appearing as a tricky scenario where an ACL seems the obvious culprit but the real issue is the reverse path check. A common trap is forgetting that uRPF strict mode requires symmetric routing on the tunnel interface itself. Memory tip: "Strict mode needs the same road back—if the route doesn't point to the tunnel, the packet gets the boot."
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 network engineer is troubleshooting an IPv6 connectivity issue on a router that is using a tunnel interface (IPv6 over IPv4). The engineer notices that traffic is not passing through the tunnel. The engineer checks the tunnel interface and finds an inbound IPv6 ACL that permits only certain IPv6 traffic. The engineer also sees that uRPF is enabled on the tunnel interface in strict mode. The tunnel source and destination are IPv4 addresses. The IPv6 traffic sourced from a network behind the tunnel is being dropped. 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 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 uRPF strict mode check fails because the router does not have a route to the source IPv6 network pointing to the tunnel interface.
The uRPF strict mode check requires that the source IPv6 address of incoming traffic on the tunnel interface must have a route in the routing table pointing back to that same interface. Since the IPv6 traffic is sourced from a network behind the tunnel, the router likely has a route to that source network via a different interface (e.g., the physical LAN interface) or no route at all, causing uRPF to drop the packets. This is the most likely cause because the tunnel interface is the inbound interface for the decapsulated IPv6 packets, and uRPF strict mode validates the source address against the Forwarding Information Base (FIB) entry pointing to the tunnel interface.
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 uRPF strict mode check fails because the router does not have a route to the source IPv6 network pointing to the tunnel interface.
Why this is correct
Correct because uRPF on a tunnel requires a route to the source via the tunnel interface; without it, the packet 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 ACL is blocking the IPv6 traffic because the tunnel interface does not support ACLs.
Why it's wrong here
Incorrect because ACLs can be applied to tunnel interfaces.
- ✗
The tunnel is not configured with the correct IPv4 source and destination.
Why it's wrong here
Incorrect because the tunnel is up and traffic is being received, indicating the tunnel configuration is correct.
- ✗
The uRPF mode should be loose mode to allow traffic from any source.
Why it's wrong here
Incorrect because loose mode would still require a route to the source, but the issue is the route pointing to the tunnel interface; loose mode might work if there is any route, but the scenario implies strict mode is the problem.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the interaction between uRPF and tunnel interfaces, where candidates mistakenly think the ACL is the issue or that uRPF only checks for the existence of a route, ignoring the strict mode requirement that the route must point back to the same interface the packet arrived on.
Trap categories for this question
Scenario analysis trap
Incorrect because loose mode would still require a route to the source, but the issue is the route pointing to the tunnel interface; loose mode might work if there is any route, but the scenario implies strict mode is the problem.
Detailed technical explanation
How to think about this question
uRPF strict mode (RFC 3704) verifies that the source IP address of a packet has a FIB entry with the same incoming interface as the one the packet arrived on. On a tunnel interface, the decapsulated IPv6 packet is treated as arriving on the tunnel interface, so the router must have a route to the source IPv6 network pointing out the tunnel interface (e.g., via a static route or dynamic routing protocol over the tunnel). In real-world scenarios, this often occurs when the tunnel is used for site-to-site connectivity but the routing table has a default route or more specific route via the physical interface, causing uRPF to drop legitimate 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.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The uRPF strict mode check fails because the router does not have a route to the source IPv6 network pointing to the tunnel interface. — The uRPF strict mode check requires that the source IPv6 address of incoming traffic on the tunnel interface must have a route in the routing table pointing back to that same interface. Since the IPv6 traffic is sourced from a network behind the tunnel, the router likely has a route to that source network via a different interface (e.g., the physical LAN interface) or no route at all, causing uRPF to drop the packets. This is the most likely cause because the tunnel interface is the inbound interface for the decapsulated IPv6 packets, and uRPF strict mode validates the source address against the Forwarding Information Base (FIB) entry pointing to the tunnel interface.
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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Last reviewed: Jun 24, 2026
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