- A
The packet will be dropped because strict uRPF requires the return path to be through the same interface.
Correct. Strict uRPF checks that the best route to the source uses the same interface as the packet arrived on.
- B
The packet will be forwarded because a route exists.
Why wrong: Incorrect. Strict uRPF also checks the interface.
- C
The packet will be forwarded because uRPF is loose.
Why wrong: Incorrect. The debug does not specify loose; strict is implied by interface check.
- D
The router will change the route to use GigabitEthernet0/0.
Why wrong: Incorrect. uRPF does not modify routing.
Question 1,904 of 2,152
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 runs the following command to debug IPv6 uRPF with detailed information:
R1# debug ipv6 verify detail
IPv6 verify debugging is on (detail)
*Mar 1 00:03:45.678: IPv6 verify: source 2001:DB8:5::1 on GigabitEthernet0/0 *Mar 1 00:03:45.678: route to source via GigabitEthernet0/1, not same as input interface
What does this output indicate?
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 packet will be dropped because strict uRPF requires the return path to be through the same interface.
The debug output shows that the source address 2001:DB8:5::1 is reachable via GigabitEthernet0/1, but the packet arrived on GigabitEthernet0/0. With strict unicast Reverse Path Forwarding (uRPF), the router verifies that the best return route to the source uses the same interface on which the packet was received. Since the interfaces do not match, the router drops the packet to prevent spoofing.
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 packet will be dropped because strict uRPF requires the return path to be through the same interface.
Why this is correct
Correct. Strict uRPF checks that the best route to the source uses the same interface as the packet arrived on.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The packet will be forwarded because a route exists.
Why it's wrong here
Incorrect. Strict uRPF also checks the interface.
- ✗
The packet will be forwarded because uRPF is loose.
Why it's wrong here
Incorrect. The debug does not specify loose; strict is implied by interface check.
- ✗
The router will change the route to use GigabitEthernet0/0.
Why it's wrong here
Incorrect. uRPF does not modify routing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates assume any valid route means the packet is forwarded, ignoring the critical interface match requirement for strict mode.
Detailed technical explanation
How to think about this question
Strict uRPF is defined in RFC 3704 and is commonly used to mitigate source IP address spoofing in networks with symmetrical routing. The router performs a FIB lookup on the source address and checks that the outgoing interface matches the ingress interface; if they differ, the packet is dropped. In contrast, loose uRPF only requires a route to the source in the FIB, making it suitable for asymmetric routing environments like multihomed connections.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
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IPv6 Traffic Filtering and uRPF — study guide chapter
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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 packet will be dropped because strict uRPF requires the return path to be through the same interface. — The debug output shows that the source address 2001:DB8:5::1 is reachable via GigabitEthernet0/1, but the packet arrived on GigabitEthernet0/0. With strict unicast Reverse Path Forwarding (uRPF), the router verifies that the best return route to the source uses the same interface on which the packet was received. Since the interfaces do not match, the router drops the packet to prevent spoofing.
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.
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Last reviewed: Jul 4, 2026
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