The answer is that the application layer is failing to establish a proper connection. VPC Flow Logs capture only network-layer metadata—such as source/destination IP, port, protocol, and the ACCEPT or REJECT action—so an ACCEPT record confirms that security groups, network ACLs, and routing are all permitting the traffic. However, flow logs cannot see beyond the network layer; they do not log TLS handshake failures, certificate mismatches, or whether the target application is actually listening on port 443. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to distinguish between network-layer connectivity and application-layer health, a common trap where engineers mistakenly blame security groups or ACLs when the logs show ACCEPT. Remember the mnemonic: “Flow logs show the road is open, but they can’t tell you if anyone is home at the application layer.”
ANS-C01 Network Implementation Practice Question
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.
```
VPC Flow Logs version 2
account-id 123456789012
interface-id eni-0a1b2c3d4e5f67890
srcaddr 10.0.1.10
dstaddr 10.0.2.10
srcport 12345
dstport 443
protocol 6
packets 10
bytes 1500
start 1625097600
end 1625097660
action ACCEPT
log-status OK
```
A network engineer is analyzing VPC Flow Logs for a VPC with CIDR 10.0.0.0/16. The exhibit shows a sample log entry. The engineer notices that traffic from 10.0.1.10 to 10.0.2.10 on port 443 is being accepted. However, the application team reports that the connection is failing. What is the most likely reason for the disconnect?
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 application layer is failing to establish a proper connection.
The VPC Flow Logs show that traffic from 10.0.1.10 to 10.0.2.10 on port 443 is being accepted (ACCEPT record), which indicates that the network layer (security groups, network ACLs, and routing) is permitting the traffic. Since the application team reports the connection is failing, the issue must be at a higher layer, specifically the application layer (e.g., TLS handshake failure, certificate mismatch, or the application not listening on port 443). Flow logs capture only network-layer metadata (IP, port, protocol, action) and cannot detect application-layer errors.
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 security groups are blocking the traffic.
Why it's wrong here
The flow log action is ACCEPT, indicating no block.
✗
The route tables are incorrectly configured, causing packet loss.
Why it's wrong here
If packets were lost, the action would likely be REJECT or not logged.
✗
The flow logs are misconfigured and not capturing all traffic.
Why it's wrong here
The log shows traffic was captured.
✓
The application layer is failing to establish a proper connection.
Why this is correct
Flow logs only show network-level acceptance; application issues are not captured.
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
AWS often tests the distinction between network-layer acceptance (ACCEPT in flow logs) and application-layer success, trapping candidates who assume an ACCEPT record guarantees a successful connection.
Trap categories for this question
Command / output trap
The log shows traffic was captured.
Detailed technical explanation
How to think about this question
VPC Flow Logs capture metadata about IP traffic going to and from network interfaces in a VPC, including the action (ACCEPT or REJECT) based on security group and network ACL evaluation. However, they do not inspect payload contents, so application-layer issues such as a missing listener, TLS handshake failure, or HTTP 500 errors are invisible to flow logs. In real-world scenarios, a common cause is that the target instance's application (e.g., a web server) is not running or is bound to a different port, causing TCP SYN packets to be accepted at the network layer but the application never completing the handshake.
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.
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 application layer is failing to establish a proper connection. — The VPC Flow Logs show that traffic from 10.0.1.10 to 10.0.2.10 on port 443 is being accepted (ACCEPT record), which indicates that the network layer (security groups, network ACLs, and routing) is permitting the traffic. Since the application team reports the connection is failing, the issue must be at a higher layer, specifically the application layer (e.g., TLS handshake failure, certificate mismatch, or the application not listening on port 443). Flow logs capture only network-layer metadata (IP, port, protocol, action) and cannot detect application-layer errors.
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: "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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