CCNA Network Services and Security Practice Question
A network engineer notices that after removing a standard ACL that was applied inbound on the internet-facing interface, the router is now receiving IP packets from the internet with source IP addresses in the 10.0.0.0/8 range, which were previously blocked. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that removing an ACL from an interface leaves some filtering in place, when in fact it restores the default permit-all behavior, and candidates may confuse the implicit deny of an ACL with the default interface behavior.
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
✓
Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.
When a standard ACL is removed from an interface using the 'no ip access-group' command, the implicit deny any at the end of the ACL is also removed. By default, Cisco routers permit all IP traffic on an interface unless an ACL is applied to filter it. Therefore, after removal, the router reverts to its default permit-all behavior, allowing previously blocked traffic like source IPs in the 10.0.0.0/8 range.
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 original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.
Why it's wrong here
The misunderstanding here is that an ACL has effect merely by being defined in the configuration. In Cisco IOS, an ACL is only a filter when it is bound to an interface via the ip access-group command. Removing that binding deactivates the ACL entirely, leaving the permit statement in the running configuration as an inert list that does not inspect or affect any traffic. The router then applies its default permit-all behavior to that interface.
- ✗
The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.
Why it's wrong here
This option assumes a half-removed state where the ip access-group reference lingers after the ACL itself is deleted. In practice, removing the ACL from an interface also removes the ip access-group command, so no dangling reference remains. Even if such a reference were possible, Cisco IOS would not fall back to selectively permitting 10.0.0.0/8 based on the old ACL's contents; the system does not infer prior rules into a default deny. The router's behavior is simply all-pass once the ACL is de-applied.
- ✓
Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.
Why this is correct
Before removal, the applied ACL permitted only 10.0.0.0/8 and denied everything else (implicit deny all), which correctly blocked spoofed RFC 1918 traffic. Once the ACL is de-applied, the interface has no access list, so all traffic is permitted, including the previously blocked spoofed packets.
- ✗
The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.
Why it's wrong here
This scenario is explicit that the ACL was removed, not reapplied in a different direction. An outbound ACL would filter only packets leaving the interface, and it would have no effect on incoming spoofed traffic, so it cannot explain why such traffic is now allowed. Without an inbound access-group, the interface reverts to the default permit-all behavior for ingress packets. The root cause is de-applying the ACL, not a mistaken change of direction.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.Correct answer▾
Why this is correct
Before removal, the applied ACL permitted only 10.0.0.0/8 and denied everything else (implicit deny all), which correctly blocked spoofed RFC 1918 traffic. Once the ACL is de-applied, the interface has no access list, so all traffic is permitted, including the previously blocked spoofed packets.
✗The original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that the ACL itself, if still configured, continues to filter traffic even when not applied to an interface.
✗The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.Wrong answer — click to see why▾
Why this is wrong here
A common misunderstanding is that the access-group line can persist without a valid ACL and cause some default behavior; in fact the entire command is removed.
✗The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.Wrong answer — click to see why▾
Why this is wrong here
Some candidates may confuse direction changes with removal and assume the ACL is still filtering traffic in some way, but the symptom clearly indicates no filtering at all.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Configuring AAA with TACACS+ and RADIUS on IOS XE
Key term
ACL
An Access Control List is a set of rules that determines who or what can access specific network resources or data.
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.