The answer is an ALB idle timeout set too low, causing 504 errors when the application takes longer to respond than the timeout allows. This occurs because the Application Load Balancer maintains a connection to the target only for the duration of its idle timeout setting; if the backend EC2 instance does not return data within that window—for example, during peak hours when processing slows—the ALB terminates the connection and returns a 504 Gateway Timeout to the client. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that flow logs showing ACCEPT do not rule out application-layer timeouts, and that health checks passing on port 8080/health only confirm the target is alive, not that it responds quickly enough under load. A common trap is to blame security groups or NACLs, but those are stateful and the logs already confirm allowed traffic. Memory tip: think of the ALB idle timeout as a “patience timer”—if your app takes longer to reply than the timer allows, the ALB gives up and sends a 504.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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.
A company runs a critical web application on EC2 instances behind an Application Load Balancer (ALB) in a VPC. The application experiences intermittent timeouts during peak hours. The network team suspects that the security group or network ACL is misconfigured. They enable VPC Flow Logs and notice that outbound traffic from the ALB to the EC2 instances on port 8080 shows 'ACCEPT' records, but the ALB returns 504 errors. The ALB health check is configured to hit the EC2 instances on port 8080/health. What is the most likely cause of the 504 errors?
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 ALB idle timeout is set too low, and the application takes longer to respond.
Option C is correct. ALB requires idle timeout settings on both client and target sides; if the target response exceeds the ALB idle timeout, it returns 504. Option A is incorrect because flow logs show ACCEPT. Option B is incorrect because health checks failing would cause unhealthy targets, but the issue is intermittent. Option D is incorrect because security group rules are stateful and outbound traffic is allowed.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 network ACL for the private subnet is blocking inbound traffic from the ALB.
Why it's wrong here
Flow logs show ACCEPT, so no blocking.
✗
The security group for the EC2 instances is missing an outbound rule to allow traffic from the ALB.
Why it's wrong here
Security groups are stateful; outbound is automatically allowed.
✗
The health check path is incorrectly configured, causing the ALB to mark all targets as unhealthy.
Why it's wrong here
Intermittent errors suggest not all targets are unhealthy.
✓
The ALB idle timeout is set too low, and the application takes longer to respond.
Why this is correct
ALB idle timeout default is 60 seconds; if app response exceeds that, 504 occurs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
Flow logs show ACCEPT, so no blocking.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The ALB idle timeout is set too low, and the application takes longer to respond. — Option C is correct. ALB requires idle timeout settings on both client and target sides; if the target response exceeds the ALB idle timeout, it returns 504. Option A is incorrect because flow logs show ACCEPT. Option B is incorrect because health checks failing would cause unhealthy targets, but the issue is intermittent. Option D is incorrect because security group rules are stateful and outbound traffic is allowed.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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