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Question 27 of 1,705

Network Implementation →mediumMultiple ChoiceObjective-mapped

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.

A company deployed an Application Load Balancer (ALB) in front of a fleet of EC2 instances. Users report intermittent timeouts. The ALB's target group health checks are failing for some instances. The instances are in private subnets with a single NAT Gateway. 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.

A
The security group on the EC2 instances does not allow inbound traffic from the ALB's security group.
ALB health checks originate from the ALB's security group; if not allowed, health checks fail.
B
The route tables in the private subnets do not have a default route pointing to the NAT Gateway.
Why wrong: Missing default route affects outbound internet traffic, not health checks from ALB.
C
The EC2 instances are of an unsupported instance type for ALB target groups.
Why wrong: ALB supports all current EC2 instance types.
D
The NAT Gateway is in a single Availability Zone, causing traffic to fail during an AZ outage.
Why wrong: Health checks do not traverse the NAT Gateway; they are sent directly from the ALB.

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 security group on the EC2 instances does not allow inbound traffic from the ALB's security group.

The most likely cause is that the security group on the EC2 instances does not allow inbound traffic from the ALB's security group. ALB health checks originate from the ALB's private IP addresses, and if the instance security group lacks an inbound rule permitting traffic from the ALB's security group (or its CIDR) on the health check port, the health checks will fail, causing intermittent timeouts as the ALB stops routing traffic to unhealthy targets.

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 group on the EC2 instances does not allow inbound traffic from the ALB's security group.
Why this is correct
ALB health checks originate from the ALB's security group; if not allowed, health checks fail.
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.
✗
The route tables in the private subnets do not have a default route pointing to the NAT Gateway.
Why it's wrong here
Missing default route affects outbound internet traffic, not health checks from ALB.
✗
The EC2 instances are of an unsupported instance type for ALB target groups.
Why it's wrong here
ALB supports all current EC2 instance types.
✗
The NAT Gateway is in a single Availability Zone, causing traffic to fail during an AZ outage.
Why it's wrong here
Health checks do not traverse the NAT Gateway; they are sent directly from the ALB.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The ANS-C01 exam often tests the misconception that NAT Gateway configuration affects internal VPC traffic, leading candidates to incorrectly select route table or NAT Gateway issues when the actual problem is security group misconfiguration for ALB health checks.

Detailed technical explanation

How to think about this question

Under the hood, ALB health checks are sent from the ALB's elastic network interfaces (ENIs) in the VPC subnets, using the source IP addresses of those ENIs. The security group on the target instances must have an inbound rule allowing traffic from the ALB's security group (or its CIDR) on the health check port and protocol (e.g., HTTP/HTTPS). A common real-world scenario is when security group rules are misconfigured to allow traffic only from the client CIDR but not from the ALB, causing health checks to fail even though user traffic might occasionally succeed if the ALB's IPs are within the client CIDR range.

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

An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.

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.

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FAQ

Questions learners often ask

What does this ANS-C01 question test?

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 security group on the EC2 instances does not allow inbound traffic from the ALB's security group. — The most likely cause is that the security group on the EC2 instances does not allow inbound traffic from the ALB's security group. ALB health checks originate from the ALB's private IP addresses, and if the instance security group lacks an inbound rule permitting traffic from the ALB's security group (or its CIDR) on the health check port, the health checks will fail, causing intermittent timeouts as the ALB stops routing traffic to unhealthy targets.

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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This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.