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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 an AWS Network Firewall in a centralized inspection VPC to inspect traffic between VPCs connected to an AWS Transit Gateway. The architecture uses Transit Gateway route tables to send inter-VPC traffic through the inspection VPC. The Network Firewall is configured with stateful and stateless rule groups. After deployment, the security team notices that traffic from VPC A to VPC B is being dropped. Other traffic flows correctly. 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 route table for VPC A's subnets does not have a route to the inspection VPC.
Why wrong: Incorrect. If the route table for VPC A's subnets lacked a route to the inspection VPC, no inter-VPC traffic would be forwarded through the firewall. Since other flows work correctly, routing is properly configured, ruling out this option.
B
The stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic.
Correct. AWS Network Firewall processes stateless rules first. If the stateless rule group is set to forward traffic, the stateful rule group must explicitly allow it. If the stateful rule group has no matching allow rule, the traffic is dropped, which explains why only traffic from VPC A to VPC B is affected.
C
The stateful rule group is configured to drop all traffic that is not explicitly allowed.
Why wrong: Incorrect. While a stateful rule group can be configured to drop all traffic not explicitly allowed, this would apply to all traffic flows, not just VPC A to VPC B. The specific nature of the drop suggests a more targeted issue, such as the stateful rule group lacking an allow rule for that particular flow.
D
The security group attached to the Transit Gateway is blocking the traffic.
Why wrong: Incorrect. Transit Gateway does not have security groups. Security groups are attached to Elastic Network Interfaces (ENIs) of resources within VPCs, not to the transit gateway itself. Therefore, security groups cannot block traffic at the transit gateway level.

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 stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic.

Option B is correct because in AWS Network Firewall, stateless rule groups evaluate traffic first. If the stateless rule group is set to 'forward' (i.e., pass traffic to the stateful engine), the stateful rule group must then explicitly allow the traffic. If the stateful rule group is not configured to allow the traffic (e.g., it has no matching allow rule or has a default drop), the traffic will be dropped. This explains why only traffic from VPC A to VPC B is dropped while other flows work, as the issue is specific to the stateful rule group's handling of that particular flow.

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 route table for VPC A's subnets does not have a route to the inspection VPC.
Why it's wrong here
Incorrect. If the route table for VPC A's subnets lacked a route to the inspection VPC, no inter-VPC traffic would be forwarded through the firewall. Since other flows work correctly, routing is properly configured, ruling out this option.
✓
The stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic.
Why this is correct
Correct. AWS Network Firewall processes stateless rules first. If the stateless rule group is set to forward traffic, the stateful rule group must explicitly allow it. If the stateful rule group has no matching allow rule, the traffic is dropped, which explains why only traffic from VPC A to VPC B is affected.
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 stateful rule group is configured to drop all traffic that is not explicitly allowed.
Why it's wrong here
Incorrect. While a stateful rule group can be configured to drop all traffic not explicitly allowed, this would apply to all traffic flows, not just VPC A to VPC B. The specific nature of the drop suggests a more targeted issue, such as the stateful rule group lacking an allow rule for that particular flow.
✗
The security group attached to the Transit Gateway is blocking the traffic.
Why it's wrong here
Incorrect. Transit Gateway does not have security groups. Security groups are attached to Elastic Network Interfaces (ENIs) of resources within VPCs, not to the transit gateway itself. Therefore, security groups cannot block traffic at the transit gateway level.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume stateless rules are the sole cause of dropped traffic, overlooking that stateful rule groups have their own default actions (typically drop) that can silently block forwarded traffic, especially when the stateless rule group is set to 'forward' but the stateful rules are incomplete.

Detailed technical explanation

How to think about this question

AWS Network Firewall processes traffic in a two-stage pipeline: stateless rule groups (evaluated first, with actions like 'forward', 'drop', or 'pass') and then stateful rule groups (evaluated only for traffic that is forwarded). The stateful engine uses Suricata-compatible rules and supports default actions (e.g., 'drop' or 'pass') for traffic that does not match any rule. In this scenario, the stateless rule group likely forwards the traffic, but the stateful rule group lacks an explicit allow rule for the VPC A-to-VPC B flow, causing the default drop action to take effect. This is a common misconfiguration when deploying centralized inspection architectures with Transit Gateway.

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.

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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 stateless rule group is set to forward traffic, but the stateful rule group is not configured to allow the traffic. — Option B is correct because in AWS Network Firewall, stateless rule groups evaluate traffic first. If the stateless rule group is set to 'forward' (i.e., pass traffic to the stateful engine), the stateful rule group must then explicitly allow the traffic. If the stateful rule group is not configured to allow the traffic (e.g., it has no matching allow rule or has a default drop), the traffic will be dropped. This explains why only traffic from VPC A to VPC B is dropped while other flows work, as the issue is specific to the stateful rule group's handling of that particular flow.

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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