PCNE Implementing a Virtual Private Cloud Practice Question
A company has a multi-region VPC with subnets in us-central1 (10.0.0.0/24) and europe-west1 (10.0.1.0/24). They have deployed a global Application Load Balancer (ALB) with backend services in both regions. The backends are instance groups with instances in each subnet. The ALB uses internal IP addresses from a subnet in each region for the backend services. The company wants to restrict access to the ALB so that only traffic from a specific list of external IP addresses (e.g., corporate VPN) can reach the load balancer. They have created a firewall rule allowing ingress from those IP addresses to the ALB's forwarding rule IP (which is a global IP). However, external traffic from allowed IPs is still being blocked. What is the most likely reason?
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 firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist.
The Global Application Load Balancer (external) uses a global anycast IP that is part of Google's frontend infrastructure, not a VPC resource. VPC firewall rules apply only to traffic entering or leaving VPC network interfaces, such as VM instances or internal load balancers. Traffic to the global ALB's frontend IP is handled before it reaches the VPC, so VPC firewall rules cannot filter it. To restrict access based on source IP addresses, you must use Cloud Armor security policies attached to the load balancer. Therefore, option B is correct. Option A is incorrect because priority is not the issue; firewall rules simply do not apply to the global frontend. Option C and D are incorrect because they refer to backend instances, but the problem is at the frontend. Option B is the correct solution: use Cloud Armor with an IP whitelist.
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 firewall rule should be created with priority lower than 1000 to override default deny.
Why it's wrong here
Default deny egress is not blocking this; the issue is that firewall doesn't apply to global LB frontend.
- ✓
VPC firewall rules do not apply to global load balancer's frontend; use Cloud Armor with IP whitelist.
Why this is correct
Global ALB traffic bypasses VPC firewall; Cloud Armor provides access control at the edge.
- ✗
The firewall rule must allow ingress from the allowed IPs to the backend instance's internal IPs.
Why it's wrong here
Traffic from the ALB to backend comes from Google's internal IPs, not the client IPs.
- ✗
The firewall rule must be applied to the backend instances' tags, not the forwarding rule IP.
Why it's wrong here
Backend instances are not directly exposed; the ALB fronts them.
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JA
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.