PCNE Implementing a Virtual Private Cloud Practice Question
A financial services company is deploying a multi-tier application in a custom VPC with three subnets: web (10.0.1.0/24), app (10.0.2.0/24), and db (10.0.3.0/24). They use a Cloud VPN with dynamic routing (BGP) to connect to their on-premises data center (10.1.0.0/16). The on-premises network administrator reports that traffic from the web tier (10.0.1.0/24) to on-premises is working, but traffic from the app tier (10.0.2.0/24) to on-premises is failing. The company uses an Identity-Aware Proxy (IAP) for SSH access. The following configurations are in place: - Cloud Router advertises all VPC subnets via BGP. - On-premises router advertises 10.1.0.0/16. - Firewall rules allow all traffic from 10.0.0.0/16 to 10.1.0.0/16. - The app tier instances have a network tag 'app-tier' and a service account 'app-sa@project.iam.gserviceaccount.com'. - There is a firewall rule with priority 1000 that denies egress from tags 'app-tier' to 10.1.0.0/16. What is the most likely cause of the failure?
⚠ Common exam trap
Google Cloud often tests the misconception that service accounts or IAP control network-level traffic, when in reality firewall rules and routing are the only mechanisms that govern packet flow between VPC subnets and on-premises networks.
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
✓
A firewall egress rule with priority 1000 denies traffic from instances with tag 'app-tier' to 10.1.0.0/16.
The firewall egress rule with priority 1000 explicitly denies traffic from instances tagged 'app-tier' to the on-premises network (10.1.0.0/16). Since firewall rules are evaluated in order of priority (lower numbers are higher priority), and this rule has a relatively low priority number, it will override any higher-numbered (lower priority) allow rules. The fact that web tier traffic works confirms that routing and VPN are functional, isolating the issue to the egress deny rule targeting the app tier.
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 service account 'app-sa' does not have permissions to send traffic through the VPN.
Why it's wrong here
Service accounts do not control network traffic; firewall rules do.
- ✗
IAP is blocking traffic from the app tier because it is not configured for that subnet.
Why it's wrong here
IAP is used for SSH access, not for general traffic to on-premises.
- ✓
A firewall egress rule with priority 1000 denies traffic from instances with tag 'app-tier' to 10.1.0.0/16.
Why this is correct
This deny rule explicitly blocks the traffic, overriding any lower-priority allow rules.
- ✗
The Cloud Router is not advertising the 10.0.2.0/24 subnet to on-premises.
Why it's wrong here
The Cloud Router is configured to advertise all subnets, so 10.0.2.0/24 should be advertised.
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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.