- A
IPv6 ACLs are applied to interfaces using the ipv6 traffic-filter command.
Correct: The command is 'ipv6 traffic-filter' in interface configuration mode.
- B
IPv6 ACLs use wildcard masks similar to IPv4 ACLs.
Why wrong: IPv6 ACLs use prefix-length notation, not wildcard masks.
- C
The implicit deny at the end of an IPv6 ACL also blocks ICMPv6 neighbor discovery messages.
Correct: The implicit deny applies to all traffic, including ND; you must explicitly permit necessary ND messages.
- D
IPv6 ACLs automatically permit ICMPv6 neighbor discovery traffic by default.
Why wrong: There is no automatic permit; you must explicitly configure it.
- E
IPv6 ACLs support the remark keyword for documentation.
Why wrong: The 'remark' keyword is not supported in classic IOS IPv6 ACLs (it is available in some IOS-XE versions, but not universally).
Quick Answer
The answer is that the implicit deny at the end of an IPv6 ACL also blocks ICMPv6 neighbor discovery messages, making this a true statement about IPv6 ACLs. This is correct because IPv6 ACLs, unlike their IPv4 counterparts, do not include an implicit permit for neighbor discovery protocol (NDP) traffic, such as Neighbor Solicitations and Router Advertisements; the `deny ipv6 any any` rule at the end of every ACL will silently drop these critical messages unless an explicit permit statement is added. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how IPv6 ACLs differ from IPv4 ACLs in handling Layer 2 control-plane traffic, often appearing in troubleshooting scenarios where connectivity fails after applying an ACL. A common trap is assuming IPv6 ACLs behave identically to IPv4 ACLs, but remember: IPv4 ACLs implicitly permit ARP, while IPv6 ACLs do not implicitly permit NDP. Memory tip: “IPv6 ACLs deny discovery—you must permit it explicitly.”
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 IPv6 traffic filtering on a Cisco router. Which TWO statements about IPv6 ACLs are true? (Choose TWO.)
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
IPv6 ACLs are applied to interfaces using the ipv6 traffic-filter command.
Option A is correct because the `ipv6 traffic-filter` command is the Cisco IOS command used to apply an IPv6 ACL to an interface for filtering inbound or outbound traffic. This is the direct IPv6 equivalent of the `ip access-group` command used for IPv4 ACLs, and it is the only valid method for applying IPv6 ACLs to filter traffic on a Cisco router 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.
- ✓
IPv6 ACLs are applied to interfaces using the ipv6 traffic-filter command.
Why this is correct
Correct: The command is 'ipv6 traffic-filter' in interface configuration mode.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
IPv6 ACLs use wildcard masks similar to IPv4 ACLs.
Why it's wrong here
IPv6 ACLs use prefix-length notation, not wildcard masks.
- ✓
The implicit deny at the end of an IPv6 ACL also blocks ICMPv6 neighbor discovery messages.
Why this is correct
Correct: The implicit deny applies to all traffic, including ND; you must explicitly permit necessary ND messages.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
IPv6 ACLs automatically permit ICMPv6 neighbor discovery traffic by default.
Why it's wrong here
There is no automatic permit; you must explicitly configure it.
- ✗
IPv6 ACLs support the remark keyword for documentation.
Why it's wrong here
The 'remark' keyword is not supported in classic IOS IPv6 ACLs (it is available in some IOS-XE versions, but not universally).
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume IPv6 ACLs behave like IPv4 ACLs in supporting wildcard masks and remarks, or that they automatically permit essential control-plane traffic like ICMPv6 neighbor discovery, leading to incorrect selections of options B, D, or E.
Trap categories for this question
Keyword trap
The 'remark' keyword is not supported in classic IOS IPv6 ACLs (it is available in some IOS-XE versions, but not universally).
Detailed technical explanation
How to think about this question
Under the hood, IPv6 ACLs are processed in order from top to bottom, and the implicit deny any any at the end blocks all traffic, including critical ICMPv6 neighbor discovery (ND) messages (types 133-137) used for address resolution and DAD. In real-world scenarios, failing to explicitly permit these ICMPv6 types can cause complete loss of IPv6 connectivity on the interface, as ND messages are essential for Layer 2 address resolution and router discovery.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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: IPv6 ACLs are applied to interfaces using the ipv6 traffic-filter command. — Option A is correct because the `ipv6 traffic-filter` command is the Cisco IOS command used to apply an IPv6 ACL to an interface for filtering inbound or outbound traffic. This is the direct IPv6 equivalent of the `ip access-group` command used for IPv4 ACLs, and it is the only valid method for applying IPv6 ACLs to filter traffic on a Cisco router 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.
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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