Question 1mediummultiple choice
Full question →mediummultiple choiceObjective-mapped
Which statement best explains why an ACL that lacks a needed permit statement can block legitimate traffic even if no explicit deny for that traffic exists?
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
Because unmatched traffic is denied by the implicit deny at the end of the ACL.
This is correct because the implicit deny blocks traffic that is not explicitly permitted.
B
Distractor review
Because the ACL automatically converts missing permits into dynamic routes.
This is wrong because ACLs do not create routes.
C
Distractor review
Because every ACL line after the first permit is ignored.
This is wrong because ACL evaluation continues until a match is found.
D
Distractor review
Because ACLs process from bottom to top.
This is wrong because ACLs are evaluated top-down.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A common exam trap is assuming that traffic not explicitly denied by an ACL will be allowed. Many candidates overlook the implicit deny at the end of every ACL, which blocks all unmatched traffic by default. This leads to the mistaken belief that missing permit statements do not affect traffic flow. In reality, if legitimate traffic does not match any permit entry, it is blocked silently by the implicit deny. This trap causes candidates to select incorrect answers that ignore the implicit deny behavior, resulting in ACLs that unintentionally block valid network communication.
Technical deep dive
How to think about this question
Access Control Lists (ACLs) are fundamental security tools in Cisco networking used to filter traffic based on defined criteria such as source/destination IP addresses, protocols, or ports. ACLs are processed sequentially from top to bottom, evaluating each packet against the list entries until a match is found. If a packet matches a permit statement, it is allowed through; if it matches a deny statement, it is blocked. This sequential evaluation ensures precise control over network traffic flow and security enforcement. A critical rule in ACL processing is the implicit deny at the end of every ACL. This means that if a packet does not match any explicit permit or deny statement, it is automatically denied by default. Therefore, even if an ACL lacks an explicit deny for certain traffic, that traffic will be blocked unless a permit statement explicitly allows it. This implicit deny acts as a safety net to prevent unintended traffic from passing through but requires careful ACL design to avoid blocking legitimate traffic. The exam trap arises when candidates overlook the implicit deny and assume that traffic not explicitly denied will be allowed. This misunderstanding leads to ACLs that unintentionally block legitimate traffic because no permit statement matches it. In practical networking, this behavior enforces strict security but demands that all necessary traffic types be explicitly permitted. Recognizing this implicit deny is essential for both passing the CCNA exam and designing effective ACLs in real-world Cisco environments.
KKey Concepts to Remember
- ACLs evaluate packets sequentially from the top entry to the bottom entry until a matching permit or deny statement is found.
- Every ACL has an implicit deny at the end that blocks any traffic not explicitly permitted by earlier statements.
- If an ACL lacks a permit statement for specific legitimate traffic, that traffic is blocked by the implicit deny even without an explicit deny line.
- Cisco routers process ACLs in a top-down manner, stopping evaluation as soon as a match is found to optimize performance.
- An implicit deny prevents unintended traffic from passing but requires careful ACL design to avoid blocking authorized traffic.
- Explicit deny statements are optional because the implicit deny at the end enforces default blocking behavior.
- Understanding the implicit deny is crucial to avoid common ACL design mistakes that cause legitimate traffic loss.
- ACLs do not modify routing or create routes; they only filter traffic based on defined criteria.
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?
ACLs evaluate packets sequentially from the top entry to the bottom entry until a matching permit or deny statement is found.
What is the correct answer to this question?
The correct answer is: Because unmatched traffic is denied by the implicit deny at the end of the ACL. — It happens because of the implicit deny at the end of the ACL. In practical terms, if the legitimate traffic does not match an earlier permit entry, it keeps moving down the list until it eventually hits the unstated final deny. That means the traffic is blocked even though there was no visible deny line for it. This is one of the most common ACL design mistakes and one of the most important exam rules to remember.
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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