PCNE Implementing a Virtual Private Cloud Practice Question
A company wants to enable VPC Flow Logs for a subnet to troubleshoot connectivity issues. They have enabled flow logs with a sample rate of 1.0 and metadata annotation enabled. After a few hours, they notice that logs are being generated but they are missing flows from a specific application server to a database server in the same subnet. Both servers are Compute Engine instances with internal IPs only. What could be the cause?
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
✓
VPC Flow Logs do not capture traffic between instances in the same subnet because it does not traverse the VPC router.
VPC Flow Logs capture IP traffic that traverses the VPC router. Traffic between two instances within the same subnet is forwarded at layer 2, without going through the router, so it is not captured by flow logs. This is why the flows from the application server to the database server are missing. Option D correctly identifies this behavior.
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 instances are using internal DNS names, and flow logs only capture traffic by IP address.
Why it's wrong here
Flow logs capture IP traffic regardless of DNS names.
- ✗
The sample rate of 1.0 means all flows are sampled, but metadata annotation may be causing some flows to be dropped.
Why it's wrong here
Sample rate 1.0 captures all flows, and metadata annotation does not drop flows.
- ✗
Flow logs must be enabled on the VPC network, not just the subnet.
Why it's wrong here
Flow logs are enabled per subnet; they work when enabled on the subnet.
- ✓
VPC Flow Logs do not capture traffic between instances in the same subnet because it does not traverse the VPC router.
Why this is correct
Traffic within the same subnet is sent directly at layer 2, bypassing the router where flow logs are captured.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCNE practice question is part of Courseiva's free Google Cloud 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 PCNE exam.