The answer is a missing permit for necessary traffic, such as return traffic or ARP, which triggers the implicit deny. When an extended ACL is applied inbound on an interface, it filters all traffic entering that interface before the routing decision, so even if you permit the destination server’s IP and required ports, you must also explicitly permit the return traffic—like TCP acknowledgments or ICMP replies—from the server back to the users. Without those permits, the implicit deny at the end of the ACL drops that return traffic, breaking connectivity and causing the reported traffic loss. On the Cisco SCOR / CCNP Security Core 350-701 exam, this scenario tests your understanding that ACLs are stateful only if you configure them to be; a common trap is assuming a permit for the destination automatically allows the reply. A helpful memory tip is “permit both ways or lose the conversation”—always account for return traffic and layer 2 protocols like ARP when troubleshooting extended ACL implicit deny traffic loss.
350-701 Security Concepts Practice Question
This 350-701 practice question tests your understanding of security concepts. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
After applying a new extended ACL inbound on an interface, users report they can no longer reach a critical server on a different subnet. The ACL permits the server's IP and required ports. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is missing a permit for necessary traffic (e.g., return traffic or ARP), triggering the implicit deny.
When an extended ACL is applied inbound on an interface, it filters traffic entering that interface before the routing decision. Even if the ACL permits the destination server's IP and required ports, it must also permit the return traffic (e.g., TCP acknowledgments, ICMP replies) from the server back to the users. If the ACL does not explicitly permit this return traffic, the implicit deny at the end of the ACL will drop it, breaking connectivity. This is the most common cause of connectivity loss after applying an inbound ACL.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 router has run out of memory for ACL processing.
Why it's wrong here
Unlikely in a typical scenario.
✗
The ACL is applied in the outbound direction instead of inbound.
Why it's wrong here
Direction change might cause issues but not typical for access.
✗
The ACL is applied to the wrong interface.
Why it's wrong here
Could be, but users report they can't reach server, likely ACL blocking.
✓
The ACL is missing a permit for necessary traffic (e.g., return traffic or ARP), triggering the implicit deny.
Why this is correct
Extended ACLs end with implicit deny; missing permit for other traffic blocks communication.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the concept that an inbound ACL filters traffic before the routing decision, and candidates mistakenly focus only on the destination server's IP and ports, forgetting that return traffic must also be explicitly permitted to avoid the implicit deny.
Trap categories for this question
Scenario analysis trap
Unlikely in a typical scenario.
Detailed technical explanation
How to think about this question
Extended ACLs evaluate packets in order, and the implicit deny any at the end drops all traffic not explicitly permitted. For stateful protocols like TCP, return traffic (e.g., SYN-ACK) must be permitted by an explicit permit statement for the source IP of the server and the destination IP of the clients, or by using the 'established' keyword for TCP. In real-world scenarios, forgetting to permit ICMP unreachable messages or ARP (which is handled locally but can be affected by ACLs on some platforms) can also cause silent failures.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Security Concepts — This question tests Security Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The ACL is missing a permit for necessary traffic (e.g., return traffic or ARP), triggering the implicit deny. — When an extended ACL is applied inbound on an interface, it filters traffic entering that interface before the routing decision. Even if the ACL permits the destination server's IP and required ports, it must also permit the return traffic (e.g., TCP acknowledgments, ICMP replies) from the server back to the users. If the ACL does not explicitly permit this return traffic, the implicit deny at the end of the ACL will drop it, breaking connectivity. This is the most common cause of connectivity loss after applying an inbound ACL.
What should I do if I get this 350-701 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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