- A
The packet is denied because the source address 2001:DB8:2::1 matches the deny entry in the access list.
Correct. The debug clearly states the packet is denied by the access list, which has a deny for that source prefix.
- B
The packet is permitted because it is a TCP packet.
Why wrong: Incorrect. The debug shows it is denied regardless of protocol.
- C
The packet is denied because of uRPF check failure.
Why wrong: Incorrect. The debug explicitly says 'denied by access-list FILTER', not uRPF.
- D
The packet is permitted because the destination is not in the access list.
Why wrong: Incorrect. The access list filters based on source; the packet is denied.
Question 876 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 traffic filtering:
R1# debug ipv6 packet access-list FILTER detail
IPv6 packet debugging is on for access list FILTER (detail)
*Mar 1 00:01:23.456: IPv6: source 2001:DB8:2::1 (GigabitEthernet0/0) *Mar 1 00:01:23.456: dest 2001:DB8:3::1 (GigabitEthernet0/1) *Mar 1 00:01:23.456: traffic class 0, flowlabel 0, hlim 64, next header 6 (TCP) *Mar 1 00:01:23.456: denied by access-list FILTER
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 is denied because the source address 2001:DB8:2::1 matches the deny entry in the access list.
The debug output explicitly states 'denied by access-list FILTER', confirming that the packet was dropped due to a match against a deny entry in the named IPv6 access list. The source address 2001:DB8:2::1 is shown as the matching source, and since the debug includes 'detail', it provides the exact reason for denial. Access lists process entries sequentially, and the first match (permit or deny) determines the action; here, the source matched a deny statement.
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 is denied because the source address 2001:DB8:2::1 matches the deny entry in the access list.
Why this is correct
Correct. The debug clearly states the packet is denied by the access list, which has a deny for that source prefix.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The packet is permitted because it is a TCP packet.
Why it's wrong here
Incorrect. The debug shows it is denied regardless of protocol.
- ✗
The packet is denied because of uRPF check failure.
Why it's wrong here
Incorrect. The debug explicitly says 'denied by access-list FILTER', not uRPF.
- ✗
The packet is permitted because the destination is not in the access list.
Why it's wrong here
Incorrect. The access list filters based on source; the packet is denied.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a packet is permitted by default if the destination is not explicitly listed in the access list, but the implicit deny at the end of every IPv6 access list means any unmatched packet is denied, and the debug output here explicitly shows a deny due to a match on the source address.
Trap categories for this question
Command / output trap
Incorrect. The debug shows it is denied regardless of protocol.
Detailed technical explanation
How to think about this question
The 'debug ipv6 packet access-list FILTER detail' command enables detailed logging of IPv6 packets that match the specified access list, showing both permitted and denied packets. Under the hood, Cisco IOS processes IPv6 access lists using a sequential match logic similar to IPv4, where each entry is evaluated in order until a match is found; if no match occurs, an implicit deny all is applied at the end. In real-world scenarios, this debug is invaluable for troubleshooting traffic drops caused by misconfigured ACLs, especially when combined with 'show access-list' to verify hit counts.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Visual reference
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 packet is denied because the source address 2001:DB8:2::1 matches the deny entry in the access list. — The debug output explicitly states 'denied by access-list FILTER', confirming that the packet was dropped due to a match against a deny entry in the named IPv6 access list. The source address 2001:DB8:2::1 is shown as the matching source, and since the debug includes 'detail', it provides the exact reason for denial. Access lists process entries sequentially, and the first match (permit or deny) determines the action; here, the source matched a deny statement.
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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