easyMultiple Choice
300-410 Practice Question: Which statement is true about the implicit deny…
Which statement is true about the implicit deny any at the end of an IPv4 ACL?
⚠ Common exam trap
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.
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
✓
It denies all traffic not explicitly permitted and is always present.
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.
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 does not override the implicit deny; it matches traffic earlier in the sequential top-down evaluation, so the implicit rule is never reached. It is tempting because the observable effect looks like removal, and would be correct if ACLs were evaluated as unordered sets rather than sequentially.
- ✗
It is invisible and applies only to TCP traffic.
Why it's wrong here
An implicit deny any is invisible but matches all IP traffic, not just TCP, so it also drops UDP, ICMP and other protocols. It is tempting because ACLs are commonly associated with TCP port filtering, but the implicit deny is protocol-agnostic and applies to every packet not explicitly permitted.
- ✗
It is always present and cannot be removed or overridden.
Why it's wrong here
The implicit deny any is fixed behaviour appended to every IPv4 ACL and cannot be deleted; it can only be pre-empted by an explicit permit earlier in the list. It is tempting because administrators can add a permit any statement, which appears to override it, but that merely matches traffic before the implicit rule is evaluated.
- ✓
It denies all traffic not explicitly permitted and is always present.
Why this is correct
Every IPv4 ACL concludes with an invisible deny any statement, so any packet not matched by an explicit permit is dropped. This implicit deny is automatically appended and cannot be removed, meaning traffic must be explicitly permitted to pass.
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Same concept, more angles
1 more way this is tested on 300-410
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.