Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
What is the main effect of the implicit deny at the end of an ACL?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Best answer
Any traffic not explicitly permitted by an earlier matching ACL entry is denied.
This is correct because unmatched traffic hits the implicit deny.
B
Distractor review
Any traffic not explicitly denied is automatically permitted.
This is wrong because Cisco ACLs do not end with implicit permit.
C
Distractor review
The ACL is ignored unless a manual deny statement is added last.
This is wrong because the implicit deny exists even without an explicit final line.
D
Distractor review
Only inbound ACLs use implicit deny behavior.
This is wrong because implicit deny is a general ACL behavior, not limited to one direction.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that an ACL without an explicit deny statement at the end will permit all unmatched traffic. This misconception leads candidates to select options suggesting implicit permit behavior or the need to add a manual deny line. In reality, Cisco ACLs always have an implicit deny at the end, which blocks any traffic not explicitly permitted. Forgetting this causes unintended network outages or overblocking, as legitimate traffic is dropped silently. Candidates must remember that the implicit deny is automatic and does not require configuration, making it a crucial detail for ACL design and troubleshooting.
Technical deep dive
How to think about this question
An Access Control List (ACL) in Cisco networking is a set of rules used to filter network traffic based on criteria such as source and destination IP addresses, protocols, or ports. The implicit deny is a fundamental concept in ACLs, meaning that if a packet does not match any explicit permit statement in the ACL, it is automatically denied by default. This implicit deny rule is not written in the ACL but is always present at the end of every ACL, ensuring that only explicitly permitted traffic is allowed through. The implicit deny rule enforces a strict security posture by blocking any traffic that is not explicitly permitted. When a router or switch processes an ACL, it evaluates each packet against the ACL entries in order. If a packet matches a permit statement, it is allowed; if it matches a deny statement, it is blocked. If the packet does not match any entry, the implicit deny at the end of the ACL denies it. This behavior means network administrators must carefully design ACLs to include all necessary permit statements to avoid unintentionally blocking legitimate traffic. A common exam trap is misunderstanding the implicit deny as requiring an explicit deny statement at the end of the ACL. Many candidates mistakenly believe that if no deny statement is present, the ACL will permit all unmatched traffic, which is incorrect. In practice, the implicit deny silently blocks all unmatched traffic, which can cause unexpected connectivity issues if the ACL is incomplete. Understanding this behavior is critical for both the CCNA exam and real-world network security to prevent accidental service disruptions.
KKey Concepts to Remember
- An ACL processes packets sequentially and permits or denies traffic based on the first matching entry it encounters.
- The implicit deny rule automatically blocks any traffic that does not match an explicit permit statement in the ACL.
- Cisco ACLs do not require a final explicit deny statement because the implicit deny is always present by default.
- The implicit deny applies to all ACLs regardless of direction, affecting both inbound and outbound traffic filtering.
- Failure to include all necessary permit statements in an ACL leads to unintended blocking due to the implicit deny.
- The implicit deny enforces a default security posture by preventing any unspecified traffic from passing through.
- Understanding the implicit deny is essential for designing effective ACLs and avoiding network connectivity issues.
- The implicit deny is a key concept tested in the CCNA exam to assess knowledge of ACL behavior and security fundamentals.
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
An ACL processes packets sequentially and permits or denies traffic based on the first matching entry it encounters.
What is the correct answer to this question?
The correct answer is: Any traffic not explicitly permitted by an earlier matching ACL entry is denied. — The implicit deny means that any traffic not explicitly permitted by an earlier matching ACL entry is denied. In practical terms, the ACL does not need a written final deny-all line to have this effect. It already exists conceptually. That is why ACL design must include all required permit statements explicitly. This is one of the most important ACL rules in Cisco networking. Forgetting it leads to many unintended outages or overblocking errors.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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