- A
The ACE defaults to 'permit ipv6'.
Why wrong: There is no default protocol; the ACE must explicitly specify a protocol.
- B
The ACE is invalid and ignored.
Cisco IOS requires a protocol in each IPv6 ACL entry; without it, the entry is invalid.
- C
The ACE defaults to 'deny ipv6'.
Why wrong: No default protocol is assumed; the entry is simply invalid.
- D
The ACE matches all IPv6 traffic.
Why wrong: Only the 'ipv6' keyword matches all IPv6 traffic; omitting the protocol makes the entry invalid.
Question 1,295 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.
In IPv6, what is the default action for an access-list entry that does not specify a protocol?
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 ACE is invalid and ignored.
In IPv6, an access-list entry (ACE) that does not specify a protocol is considered invalid and is ignored by the router. This is because the IPv6 access-list syntax requires a protocol keyword (e.g., tcp, udp, icmp, or a protocol number) to define the match criteria; without it, the ACE cannot be parsed correctly and is treated as a configuration error, not a default permit or deny.
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 ACE defaults to 'permit ipv6'.
Why it's wrong here
There is no default protocol; the ACE must explicitly specify a protocol.
- ✓
The ACE is invalid and ignored.
Why this is correct
Cisco IOS requires a protocol in each IPv6 ACL entry; without it, the entry is invalid.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The ACE defaults to 'deny ipv6'.
Why it's wrong here
No default protocol is assumed; the entry is simply invalid.
- ✗
The ACE matches all IPv6 traffic.
Why it's wrong here
Only the 'ipv6' keyword matches all IPv6 traffic; omitting the protocol makes the entry invalid.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that an ACE without a protocol defaults to 'permit ipv6' or 'deny ipv6', similar to how IPv4 ACLs handle missing protocol fields, but in IPv6 the entry is simply invalid and ignored.
Trap categories for this question
Keyword trap
Only the 'ipv6' keyword matches all IPv6 traffic; omitting the protocol makes the entry invalid.
Detailed technical explanation
How to think about this question
Under the hood, Cisco IOS processes IPv6 ACL entries by parsing the protocol field as a mandatory parameter; if missing, the parser rejects the entry and logs an error (e.g., '% Invalid input detected'). This behavior is defined by the IPv6 ACL syntax in RFC 2460 and Cisco implementation, where each ACE must include a protocol or 'ipv6' to be valid. In real-world scenarios, a misconfigured ACE without a protocol can lead to unexpected traffic passing or being blocked, as the router silently ignores the faulty line, potentially creating security gaps that are hard to debug without careful configuration review.
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 ACE is invalid and ignored. — In IPv6, an access-list entry (ACE) that does not specify a protocol is considered invalid and is ignored by the router. This is because the IPv6 access-list syntax requires a protocol keyword (e.g., tcp, udp, icmp, or a protocol number) to define the match criteria; without it, the ACE cannot be parsed correctly and is treated as a configuration error, not a default permit or deny.
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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