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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 is deploying a multi-tier web application on AWS. The application consists of an Application Load Balancer (ALB), a fleet of EC2 instances in an Auto Scaling group across three Availability Zones, and an Amazon RDS for MySQL database. The ALB has a target group that routes traffic to the EC2 instances on TCP port 8080. The security group for the EC2 instances allows inbound traffic from the ALB's security group on port 8080. Users report intermittent connectivity issues to the application. A network engineer reviews the VPC Flow Logs and notices that traffic from the ALB to the EC2 instances is being recorded as 'REJECT' for some requests. What is the most likely cause of this issue?

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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.
The network ACL is stateless and must allow return traffic. Missing outbound rules cause REJECT.
B
The ALB's security group is blocking inbound traffic from the EC2 instances on the response path.
Why wrong: The ALB does not have a security group that affects traffic to targets; target security group handles inbound.
C
The ALB's target group health check is misconfigured, causing the ALB to mark instances as unhealthy and stop sending traffic.
Why wrong: Health checks use the same security group rules; if instances are healthy, traffic should flow.
D
The security group on the EC2 instances is stateful and automatically allows return traffic; the issue cannot be security group related.
Why wrong: Security groups are stateful, but network ACLs are not; the issue is likely with the ACL, not the security group.

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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.

The correct answer is A because the network ACL (NACL) is stateless and must have explicit outbound rules to allow return traffic from the EC2 instances back to the ALB on ephemeral ports. Since the ALB initiates connections to the EC2 instances on TCP port 8080, the return traffic from the EC2 instances uses ephemeral ports (typically 1024-65535) destined for the ALB's source port. Without an outbound rule in the subnet's NACL allowing this traffic, the NACL will reject the response packets, causing intermittent REJECT entries in VPC Flow Logs.

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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports.
Why this is correct
The network ACL is stateless and must allow return traffic. Missing outbound rules cause REJECT.
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 ALB's security group is blocking inbound traffic from the EC2 instances on the response path.
Why it's wrong here
The ALB does not have a security group that affects traffic to targets; target security group handles inbound.
✗
The ALB's target group health check is misconfigured, causing the ALB to mark instances as unhealthy and stop sending traffic.
Why it's wrong here
Health checks use the same security group rules; if instances are healthy, traffic should flow.
✗
The security group on the EC2 instances is stateful and automatically allows return traffic; the issue cannot be security group related.
Why it's wrong here
Security groups are stateful, but network ACLs are not; the issue is likely with the ACL, not the security group.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often focus on security groups being stateful and forget that network ACLs are stateless and require explicit rules for return traffic, especially on ephemeral ports.

Detailed technical explanation

How to think about this question

Network ACLs operate at the subnet level and are stateless, requiring explicit rules for both inbound and outbound traffic. When an ALB sends traffic to an EC2 instance on TCP port 8080, the response from the EC2 instance uses a random ephemeral port (e.g., 1024-65535) as the source port and the ALB's original source port as the destination. The subnet's NACL must have an outbound rule allowing this ephemeral port range to the ALB's subnet CIDR; otherwise, the response is dropped, causing REJECT entries in VPC Flow Logs. This is a common misconfiguration when deploying ALBs across multiple AZs with NACLs that are too restrictive.

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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.

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 network ACL associated with the EC2 instances' subnet does not have an outbound rule to allow traffic from the EC2 instances to the ALB on ephemeral ports. — The correct answer is A because the network ACL (NACL) is stateless and must have explicit outbound rules to allow return traffic from the EC2 instances back to the ALB on ephemeral ports. Since the ALB initiates connections to the EC2 instances on TCP port 8080, the return traffic from the EC2 instances uses ephemeral ports (typically 1024-65535) destined for the ALB's source port. Without an outbound rule in the subnet's NACL allowing this traffic, the NACL will reject the response packets, causing intermittent REJECT entries in VPC Flow Logs.

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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