This ANS-C01 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
CLI output from a VPC Flow Log:
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54872 6 10 1000 1432919027 1432919028 ACCEPT OK
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54873 6 25 4000 1432919028 1432919029 REJECT OK
An engineer is reviewing VPC Flow Logs for connectivity issues between two EC2 instances (10.0.1.5 and 10.0.2.10) on TCP port 443. The first log entry shows ACCEPT, the second shows REJECT. What is the most likely cause of the REJECT?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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.
Exhibit
Refer to the exhibit.
CLI output from a VPC Flow Log:
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54872 6 10 1000 1432919027 1432919028 ACCEPT OK
2 123456789010 eni-12345 10.0.1.5 10.0.2.10 443 54873 6 25 4000 1432919028 1432919029 REJECT OK
A
The destination instance has an incorrect network interface configuration.
Why wrong: Incorrect network interface configuration would likely cause a different connectivity issue, not a REJECT specifically on return traffic in VPC Flow Logs.
B
The network ACL is blocking the traffic due to an incorrect inbound rule.
Correct. Network ACLs are stateless, so they require explicit inbound rules for return traffic. The REJECT on the return packet indicates a missing or incorrect inbound NACL rule.
C
The route table for the subnet does not have a route to the destination.
Why wrong: Missing route in the route table would cause the packet to be dropped before reaching the destination, not a REJECT on the return path.
D
A security group rule is blocking the return traffic.
Why wrong: Incorrect. Security groups are stateful and automatically allow return traffic for established connections, so they would not cause a REJECT on the return packet.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network ACL is blocking the traffic due to an incorrect inbound rule.
The first log entry shows ACCEPT for the outbound SYN packet, indicating the initial connection attempt succeeded. The second REJECT indicates that the return SYN-ACK packet from 10.0.2.10 to 10.0.1.5 was dropped. Since security groups are stateful and automatically allow return traffic for established connections, they cannot cause this REJECT. Network ACLs are stateless, so they require explicit inbound rules for return traffic. If the inbound rule on the NACL for the subnet of 10.0.2.10 does not allow the return traffic (e.g., using an ephemeral port range), it will be REJECTED. Therefore, option B is correct.
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 destination instance has an incorrect network interface configuration.
Why it's wrong here
Incorrect network interface configuration would likely cause a different connectivity issue, not a REJECT specifically on return traffic in VPC Flow Logs.
✓
The network ACL is blocking the traffic due to an incorrect inbound rule.
Why this is correct
Correct. Network ACLs are stateless, so they require explicit inbound rules for return traffic. The REJECT on the return packet indicates a missing or incorrect inbound NACL rule.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The route table for the subnet does not have a route to the destination.
Why it's wrong here
Missing route in the route table would cause the packet to be dropped before reaching the destination, not a REJECT on the return path.
✗
A security group rule is blocking the return traffic.
Why it's wrong here
Incorrect. Security groups are stateful and automatically allow return traffic for established connections, so they would not cause a REJECT on the return packet.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The ANS-C01 exam often tests the nuance that asymmetric routing can cause security groups to drop return traffic. Even though security groups are stateful, if the return packet takes a different path (e.g., different subnet or ENI) than the outbound packet, the security group on that interface may not recognize it as part of an established connection and will evaluate it against inbound rules. If no appropriate inbound rule exists, the packet is REJECTED. Candidates frequently assume a REJECT on return traffic must be due to a stateless NACL, overlooking this asymmetric routing scenario.
Detailed technical explanation
How to think about this question
VPC Flow Logs capture metadata about accepted and rejected traffic at the network interface level. Security groups are stateful, meaning they automatically allow return traffic for connections that were initiated outbound and allowed; however, if a security group rule explicitly denies the return traffic (e.g., an inbound rule on the source instance that blocks the destination IP or port), the stateful tracking is overridden, causing a REJECT. This behavior is distinct from network ACLs, which are stateless and require explicit rules for both directions; a common real-world scenario is when an administrator configures a security group to allow outbound HTTPS but forgets to allow inbound ephemeral ports for the return traffic, leading to asymmetric drops.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Visual reference
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.
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The network ACL is blocking the traffic due to an incorrect inbound rule. — The first log entry shows ACCEPT for the outbound SYN packet, indicating the initial connection attempt succeeded. The second REJECT indicates that the return SYN-ACK packet from 10.0.2.10 to 10.0.1.5 was dropped. Since security groups are stateful and automatically allow return traffic for established connections, they cannot cause this REJECT. Network ACLs are stateless, so they require explicit inbound rules for return traffic. If the inbound rule on the NACL for the subnet of 10.0.2.10 does not allow the return traffic (e.g., using an ephemeral port range), it will be REJECTED. Therefore, option B is correct.
What should I do if I get this ANS-C01 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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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