ANS-C01 Network Implementation Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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.
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.
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.
- ✗
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.