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300-410 Practice Question: The default behavior of an IPv4 access control…
What is the default behavior of an IPv4 access control list (ACL) when no explicit permit or deny statement matches a packet?
⚠ Common exam trap
Cisco often tests the misconception that an ACL with no matching entries will permit traffic by default, or that the ACL is simply ignored, when in fact the implicit deny any silently drops all unmatched packets.
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.
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.
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 permitted.
Why it's wrong here
Incorrect. The default is to deny traffic that does not match any entry.
- ✓
The packet is denied.
Why this is correct
Correct. All ACLs have an implicit deny any at the end.
- ✗
The packet is logged and then permitted.
Why it's wrong here
Incorrect. Logging is only done if the 'log' keyword is used, and the default action is deny.
- ✗
The ACL is ignored and the packet is forwarded.
Why it's wrong here
Incorrect. The ACL is always applied; unmatched packets are denied.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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