← DOP-C02·Amazon Web Services

Question 1,407 of 1,740

Resilient Cloud Solutions →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is an overloaded web server failing to respond within the health check timeout period. When an ALB health check timeout occurs, it means the request reached the instance but the web server did not send back a response before the timeout threshold expired, even though the security group correctly allows inbound HTTP from the ALB. This scenario tests your understanding of the difference between a connection failure and a timeout—a key distinction on the AWS Certified DevOps Engineer Professional DOP-C02 exam, where common traps include assuming a security group misconfiguration or an incorrect health check path. Remember, a timeout indicates the instance is reachable but slow, often due to high CPU, memory pressure, or a slow application endpoint. A useful memory tip is “Timeout = Too slow, not blocked”—if the security group were the issue, you would see a connection refused error, not a timeout.

DOP-C02 Resilient Cloud Solutions Practice Question

This DOP-C02 practice question tests your understanding of resilient cloud solutions. 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.

Network Topology

A DevOps engineer is troubleshooting an issue where an EC2 instance behind an ALB target group is marked as unhealthy. The instance i-0abcd1234efgh5678 is serving traffic but the health check is timing out. The security group for the instance allows inbound HTTP from the ALB's security group. 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.

Question 1hardmultiple choice

Full question →

Network Topology

A
The instance's web server is overloaded and not responding within the health check interval.
A timeout indicates the instance is not responding in time, often due to high CPU or application hang.
B
The instance is in a private subnet with no route to the internet.
Why wrong: Health checks are initiated by the ALB within the VPC, so internet access is not required.
C
The health check path is misconfigured and returns a 404 status code.
Why wrong: A 404 would cause 'unhealthy' but with reason 'Application.Invalid' or similar, not 'Target.Timeout'.
D
The security group of the ALB does not allow outbound traffic to the instance.
Why wrong: The ALB sends health checks from its own IPs; the instance's security group must allow inbound from the ALB, which is already configured.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The instance's web server is overloaded and not responding within the health check interval.

Option D is correct because the health check is timing out, which suggests the instance is not responding within the timeout period. The instance may be busy or the health check path may be slow. Option A (security group) is already allowed. Option B (subnet) doesn't cause timeout. Option C (health check path) could be a cause if it's incorrect, but timing out indicates the request is reaching the instance but not getting a response in time.

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 instance's web server is overloaded and not responding within the health check interval.
Why this is correct
A timeout indicates the instance is not responding in time, often due to high CPU or application hang.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The instance is in a private subnet with no route to the internet.
Why it's wrong here
Health checks are initiated by the ALB within the VPC, so internet access is not required.
✗
The health check path is misconfigured and returns a 404 status code.
Why it's wrong here
A 404 would cause 'unhealthy' but with reason 'Application.Invalid' or similar, not 'Target.Timeout'.
✗
The security group of the ALB does not allow outbound traffic to the instance.
Why it's wrong here
The ALB sends health checks from its own IPs; the instance's security group must allow inbound from the ALB, which is already configured.

Common exam traps

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

Similar concept trap
A 404 would cause 'unhealthy' but with reason 'Application.Invalid' or similar, not 'Target.Timeout'.

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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DOP-C02 question test?

Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The instance's web server is overloaded and not responding within the health check interval. — Option D is correct because the health check is timing out, which suggests the instance is not responding within the timeout period. The instance may be busy or the health check path may be slow. Option A (security group) is already allowed. Option B (subnet) doesn't cause timeout. Option C (health check path) could be a cause if it's incorrect, but timing out indicates the request is reaching the instance but not getting a response in time.

What should I do if I get this DOP-C02 question wrong?

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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