- A
The uRPF loose mode check fails because there is no route to the source address in the routing table.
Correct because loose mode requires at least one route to the source address in the FIB; if no route exists, the packet is dropped.
- B
The uRPF loose mode check fails because the source address is not reachable via the same interface.
Why wrong: Incorrect because loose mode does not require the same interface; it only requires any route.
- C
The router has an ACL that blocks traffic from that source.
Why wrong: Incorrect because the symptom is specifically related to uRPF, not an ACL.
- D
The uRPF mode should be strict mode to allow the traffic.
Why wrong: Incorrect because strict mode would also drop the packet if the route does not point back to the same interface.
Quick Answer
The answer is that the uRPF loose mode check fails because there is no route to the source address in the routing table. When uRPF loose mode is enabled, the router performs a simple lookup for the source IP address in the routing table; it does not verify that the incoming interface matches the reverse path. Since the source address 2001:db8:100::1 is absent from the routing table, the loose mode check drops the packets, and a default route does not satisfy this requirement because uRPF demands an exact match for the source prefix, not a default. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that loose mode is often misconfigured when a default route exists, leading to the common trap of assuming the default will pass the check. Remember the memory tip: “Loose mode looks for the source, not the path—no exact route means no pass.”
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 a scenario where a router is dropping IPv6 packets that are destined for a server on a directly connected network. The engineer checks the interface and finds that uRPF is enabled in loose mode. The router has a default route pointing to an upstream router. The source address of the packets is 2001:db8:100::1, which is not in the routing table (the router has no route to that prefix). What is the most likely cause of the packet drops?
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 loose mode check fails because there is no route to the source address in the routing table.
With uRPF loose mode enabled, the router checks whether a route to the source address exists in the routing table, but it does not verify that the incoming interface matches the reverse path. Since the source address 2001:db8:100::1 is not present in the routing table, the loose mode check fails and the packets are dropped. The presence of a default route does not help because uRPF requires an exact match for the source prefix, not a default route.
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 loose mode check fails because there is no route to the source address in the routing table.
Why this is correct
Correct because loose mode requires at least one route to the source address in the FIB; if no route exists, 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 uRPF loose mode check fails because the source address is not reachable via the same interface.
Why it's wrong here
Incorrect because loose mode does not require the same interface; it only requires any route.
- ✗
The router has an ACL that blocks traffic from that source.
Why it's wrong here
Incorrect because the symptom is specifically related to uRPF, not an ACL.
- ✗
The uRPF mode should be strict mode to allow the traffic.
Why it's wrong here
Incorrect because strict mode would also drop the packet if the route does not point back to the same interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a default route satisfies uRPF loose mode, but in reality, uRPF requires a route to the specific source prefix, not a default route.
Detailed technical explanation
How to think about this question
uRPF loose mode (configured with 'ipv6 verify unicast source reachable-via any') only requires that the source address be present in the FIB, regardless of the incoming interface. A default route does not satisfy this requirement because uRPF looks for a prefix that matches the source address exactly or via a more specific route, not a catch-all. In real-world scenarios, this often catches spoofed traffic from non-routable or unallocated source addresses, but can also drop legitimate traffic if routing tables are incomplete.
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
Got this wrong? Here's your next step.
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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 loose mode check fails because there is no route to the source address in the routing table. — With uRPF loose mode enabled, the router checks whether a route to the source address exists in the routing table, but it does not verify that the incoming interface matches the reverse path. Since the source address 2001:db8:100::1 is not present in the routing table, the loose mode check fails and the packets are dropped. The presence of a default route does not help because uRPF requires an exact match for the source prefix, not a default route.
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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