The correct answer is that the implicit deny any at the end of an IPv4 ACL denies all traffic not explicitly permitted and is always present. This is because every standard or extended IPv4 ACL, whether numbered or named, automatically appends an invisible final entry that drops any packet not matching a preceding permit or deny statement; this rule cannot be removed or overridden by any explicit configuration. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of ACL traffic filtering behavior, often appearing in questions about troubleshooting connectivity or designing access policies—a common trap is assuming that an ACL with only permit entries will allow all traffic, when in fact any unmatched traffic is silently dropped. A reliable memory tip is to think of the implicit deny as the ACL’s “last line of defense”: if you don’t explicitly permit it, you implicitly deny it.
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
Which statement is true about the implicit deny any at the end of an IPv4 ACL?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It denies all traffic not explicitly permitted and is always present.
Option D is correct because every IPv4 ACL has an implicit deny any statement at the end that denies all traffic not explicitly permitted by earlier entries. This implicit rule is always present and cannot be removed, ensuring that only traffic matching a permit entry is allowed through the ACL.
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.
✗
It can be overridden by adding a permit any at the end.
Why it's wrong here
A permit any entry will match before the implicit deny, effectively overriding it.
✗
It is invisible and applies only to TCP traffic.
Why it's wrong here
The implicit deny applies to all IP traffic for extended ACLs.
✗
It is always present and cannot be removed or overridden.
Why it's wrong here
It can be overridden by a permit any entry placed before it.
✓
It denies all traffic not explicitly permitted and is always present.
Why this is correct
The implicit deny any is a fundamental property of Cisco ACLs.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the implicit deny any can be removed or that it only applies to specific protocols, when in fact it is a permanent, protocol-agnostic rule that denies all unmatched traffic.
Detailed technical explanation
How to think about this question
The implicit deny any is an invisible entry that Cisco IOS automatically appends to every ACL, and it is not displayed in the running configuration. In real-world scenarios, forgetting to add a permit statement for required traffic (e.g., routing protocols like OSPF or EIGRP) can cause connectivity failures, as the implicit deny will block that traffic unless explicitly permitted.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: It denies all traffic not explicitly permitted and is always present. — Option D is correct because every IPv4 ACL has an implicit deny any statement at the end that denies all traffic not explicitly permitted by earlier entries. This implicit rule is always present and cannot be removed, ensuring that only traffic matching a permit entry is allowed through the ACL.
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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. What is the default behavior of an IPv4 access control list (ACL) when no explicit permit or deny statement matches a packet?
easy
A.The packet is permitted.
✓ B.The packet is denied.
C.The packet is logged and then permitted.
D.The ACL is ignored and the packet is forwarded.
Why B: By default, IPv4 ACLs have an implicit deny any statement at the end. If a packet does not match any explicit permit or deny entry, the implicit deny any is applied, causing the packet to be dropped. This behavior is fundamental to ACL security, ensuring that only explicitly permitted traffic is allowed.
Last reviewed: Jun 24, 2026
Question Discussion
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