ANS-C01 Network Management and Operations Practice Question
A company has deployed a centralized NAT gateway in a VPC and uses VPC Flow Logs to monitor traffic. The network team notices that traffic from an EC2 instance in a private subnet to the internet is not being logged in the flow logs. The flow logs are configured at the VPC level with the 'ALL' format. What is the most likely cause of this issue?
⚠ Common exam trap
AWS often tests the misconception that VPC-level flow logs automatically capture all traffic traversing the VPC, including traffic through intermediate appliances like NAT gateways, when in fact flow logs must be applied to each specific ENI to log its traffic.
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
✓
Flow logs must be created specifically for the NAT gateway's elastic network interface to capture its traffic.
VPC Flow Logs capture traffic at the elastic network interface (ENI) level. A centralized NAT gateway has its own ENI in the public subnet, and traffic from private instances is routed through that ENI when it reaches the internet. To log that traffic, flow logs must be created specifically for the NAT gateway's ENI, not just at the VPC level, because VPC-level flow logs capture traffic for ENIs within that VPC but do not inherently capture traffic passing through the NAT gateway's ENI unless that ENI is explicitly targeted.
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 EC2 instance is using a different VPC than the one where flow logs are configured.
Why it's wrong here
The question states the instance is in a private subnet of the same VPC.
- ✗
The flow log format is set to 'ALL' which excludes NAT gateway traffic.
Why it's wrong here
The 'ALL' format includes all fields and does not exclude NAT gateway traffic.
- ✗
The flow logs are configured to capture only rejected traffic by default.
Why it's wrong here
Flow logs capture both accepted and rejected traffic based on the filter setting; default is 'ALL'.
- ✓
Flow logs must be created specifically for the NAT gateway's elastic network interface to capture its traffic.
Why this is correct
VPC-level flow logs do not capture traffic for the NAT gateway's ENI; a separate flow log for the NAT gateway ENI is required.
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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.