CCNA Network Services and Security Practice Question
Which two statements accurately describe ACL behavior on Cisco devices?
⚠ Common exam trap
A frequent exam trap is assuming that ACLs evaluate every line before deciding to permit or deny traffic. Candidates might think that if no explicit deny exists, traffic is allowed by default. However, Cisco ACLs stop processing at the first match, and if no match occurs, the implicit deny at the end blocks the traffic. This misunderstanding can lead to incorrect answers about ACL behavior and cause misconfigurations in real networks. Remembering that ACLs use a “first match wins” approach and end with an implicit deny helps avoid this trap.
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
✓
ACL entries are processed from top to bottom until a match is found.
ACL questions often depend on understanding how Cisco devices process entries line by line. In plain language, ACL entries are evaluated from top to bottom, and the first matching statement decides the result. If traffic reaches the end without matching a permit, it is denied by the implicit deny. These two ideas explain many real ACL troubleshooting cases. The wrong answers often sound plausible because they borrow language from other parts of networking. But ACLs do not normally evaluate all lines before choosing, and they do not end with an implicit permit. Once you remember “first match wins” and “implicit deny at the end,” many access-list questions become much easier.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
ACL entries are processed from top to bottom until a match is found.
Why this is correct
Cisco IOS ACLs are ordered lists; each packet is compared against the access control entries sequentially, beginning with the first line. When a packet matches an ACE, the specified permit or deny action is applied immediately, and no remaining entries are examined. This first-match behavior is fundamental to ACL design, so line order can change the outcome.
- ✓
ACLs end with an implicit deny if unmatched traffic reaches the end.
Why this is correct
If a packet traverses every entry in an ACL without matching any of them, the implicit deny at the end takes effect and silently drops the packet. This built-in final ACE is not a configured line, but it acts as 'deny ip any any' for IP ACLs. Therefore, any traffic not explicitly permitted by an earlier entry is denied by default.
- ✗
ACLs always evaluate every line before deciding the action.
Why it's wrong here
ACL processing does not require or perform a full scan of every line; it stops at the first matching entry. Once an ACE matches, the resulting permit or deny action is executed and evaluation ceases. If no match occurs, only then does the implicit deny at the end apply, but that is not a full evaluation of all lines—it is a final default.
When this WOULD be correct
In a hypothetical exam question that asks about a theoretical ACL processing model where all entries must be evaluated for logging or auditing purposes, option C could be correct, as it would imply a different operational context.
- ✗
ACLs end with an implicit permit unless configured otherwise.
Why it's wrong here
This statement is incorrect because Cisco ACLs do not contain an implicit permit; they contain an implicit deny as the final entry. Without an explicit 'permit any' ACE, any traffic that reaches the end of the ACL is discarded. The only way to permit unmatched traffic is to manually add an explicit permit-all statement before that implied deny.
When this WOULD be correct
If the exam question stated that the ACL was configured to explicitly allow all traffic at the end of the list, or if it was specifically about a different vendor's ACL implementation that defaults to an implicit permit, then this option would be correct.
- ✗
ACLs apply only to Layer 1 traffic and not IP packets.
Why it's wrong here
This is wrong because ACLs operate at Layer 3 and Layer 4 of the OSI model, filtering IP packets based on source/destination addresses and TCP/UDP port numbers. Layer 1 is the physical layer responsible for bit-level transmission over media, which is not what Cisco ACLs inspect. Input and output ACLs on routers and switches work on IP traffic, not physical signals.
When this WOULD be correct
In a different question that specifically asks about the behavior of ACLs in a network that only handles Layer 1 traffic, such as a scenario involving MAC address filtering on a switch, this option could be correct. For example, if the question was about access control methods for non-IP traffic, then this statement might apply.
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.
✓ACL entries are processed from top to bottom until a match is found.Correct answer▾
Why this is correct
Cisco IOS ACLs are ordered lists; each packet is compared against the access control entries sequentially, beginning with the first line. When a packet matches an ACE, the specified permit or deny action is applied immediately, and no remaining entries are examined. This first-match behavior is fundamental to ACL design, so line order can change the outcome.
✗ACLs always evaluate every line before deciding the action.Wrong answer — click to see why▾
Why this is wrong here
This statement is incorrect because ACLs stop processing as soon as a match is found; they do not evaluate every line. This sequential processing is fundamental to ACL behavior and ensures efficiency.
★ When this WOULD be the correct answer
In a hypothetical exam question that asks about a theoretical ACL processing model where all entries must be evaluated for logging or auditing purposes, option C could be correct, as it would imply a different operational context.
Why candidates choose this
Students might think that ACLs evaluate all entries to ensure comprehensive filtering, similar to how some firewall rules work. However, Cisco ACLs use first-match logic, not last-match.
✗ACLs end with an implicit permit unless configured otherwise.Wrong answer — click to see why▾
Why this is wrong here
This is incorrect because Cisco ACLs end with an implicit deny, not an implicit permit. An implicit permit would allow all unmatched traffic, which defeats the purpose of access control and is a security risk.
★ When this WOULD be the correct answer
If the exam question stated that the ACL was configured to explicitly allow all traffic at the end of the list, or if it was specifically about a different vendor's ACL implementation that defaults to an implicit permit, then this option would be correct.
Why candidates choose this
Some students may confuse ACL behavior with routing protocols or other features that have implicit permits. Additionally, the concept of 'implicit permit' exists in some contexts (e.g., IPv6 ACLs have an implicit permit for certain traffic), but standard ACLs have an implicit deny.
✗ACLs apply only to Layer 1 traffic and not IP packets.Wrong answer — click to see why▾
Why this is wrong here
This is incorrect because ACLs operate at Layer 3 (IP) and Layer 4 (TCP/UDP) of the OSI model, filtering based on source/destination IP addresses, ports, and protocols. They do not filter Layer 1 traffic like physical signals.
★ When this WOULD be the correct answer
In a different question that specifically asks about the behavior of ACLs in a network that only handles Layer 1 traffic, such as a scenario involving MAC address filtering on a switch, this option could be correct. For example, if the question was about access control methods for non-IP traffic, then this statement might apply.
Why candidates choose this
Students might think ACLs apply to all layers due to the term 'access control,' but ACLs are specifically designed for network layer filtering. Layer 1 filtering is handled by hardware features like port security or physical access controls.
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?”
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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Key term
Software-defined Wide Area Network
A Software-defined Wide Area Network (SD-WAN) is a virtual network architecture that uses software to manage and optimize the connections between branch offices and data centers across a wide geographic area.
Key term
ACL
An Access Control List is a set of rules that determines who or what can access specific network resources or data.
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JA
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.