- A
The packet will be dropped because uRPF cannot find a route to the source address.
Correct. uRPF requires a route to the source; if none exists, the packet is dropped.
- B
The packet will be forwarded because uRPF only checks the destination.
Why wrong: Incorrect. uRPF checks the source address.
- C
The packet will be forwarded because the source is on the same interface.
Why wrong: Incorrect. Even if on the same interface, a route is required.
- D
The router will add a route to the source address.
Why wrong: Incorrect. uRPF does not add routes; it only verifies.
Question 759 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. 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 runs the following command to debug IPv6 uRPF:
R1# debug ipv6 verify
IPv6 verify debugging is on
*Mar 1 00:02:34.567: IPv6 verify: source 2001:DB8:4::1 on GigabitEthernet0/0 *Mar 1 00:02:34.567: no route to source
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 uRPF cannot find a route to the source address.
The debug output shows that uRPF (unicast Reverse Path Forwarding) checked the source address 2001:DB8:4::1 on the incoming interface GigabitEthernet0/0 and found no route to that source in the IPv6 routing table. When uRPF is enabled and a packet arrives on an interface, the router performs a reverse path lookup: it checks whether the best return route to the source address points back to the same interface. If no route exists at all, the lookup fails, and uRPF considers the source as unreachable, causing the packet to be dropped. This is a strict-mode uRPF behavior, and the debug message 'no route to source' confirms the packet will be discarded.
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 uRPF cannot find a route to the source address.
Why this is correct
Correct. uRPF requires a route to the source; if none exists, the packet is dropped.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The packet will be forwarded because uRPF only checks the destination.
Why it's wrong here
Incorrect. uRPF checks the source address.
- ✗
The packet will be forwarded because the source is on the same interface.
Why it's wrong here
Incorrect. Even if on the same interface, a route is required.
- ✗
The router will add a route to the source address.
Why it's wrong here
Incorrect. uRPF does not add routes; it only verifies.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that uRPF checks the destination address or that it only verifies the source is on the same interface without requiring a route, but the key trap here is that 'no route to source' means the packet is dropped, not forwarded, because uRPF cannot validate the source without a matching route in the routing table.
Detailed technical explanation
How to think about this question
IPv6 uRPF relies on the FIB (Forwarding Information Base) to perform the reverse path lookup; if the source address is not present in the FIB at all, the packet is dropped even if the source is topologically valid but the route is missing due to a routing protocol issue or incomplete configuration. In strict mode, the router also verifies that the best return route points to the exact incoming interface, while in loose mode it only checks that a route to the source exists anywhere. The debug ipv6 verify command is a powerful troubleshooting tool that reveals the exact reason for uRPF failure, such as 'no route to source' or 'wrong interface'.
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 uRPF cannot find a route to the source address. — The debug output shows that uRPF (unicast Reverse Path Forwarding) checked the source address 2001:DB8:4::1 on the incoming interface GigabitEthernet0/0 and found no route to that source in the IPv6 routing table. When uRPF is enabled and a packet arrives on an interface, the router performs a reverse path lookup: it checks whether the best return route to the source address points back to the same interface. If no route exists at all, the lookup fails, and uRPF considers the source as unreachable, causing the packet to be dropped. This is a strict-mode uRPF behavior, and the debug message 'no route to source' confirms the packet will be discarded.
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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