The answer is to use VPC firewall rules with target tags to restrict network access between tiers. This is correct because target tags allow you to apply granular ingress and egress controls to specific VM instances based on their role, such as 'web-tier' or 'app-tier', rather than relying on IP ranges or subnets. By creating a firewall rule that permits traffic only from the web tier to the application tier on a designated port, and another rule from the application tier to the database tier, you enforce least privilege within a single VPC network without breaking isolation. On the Google Professional Cloud Architect exam, this scenario tests your understanding of how to implement tiered access controls without using separate VPCs or complex network appliances—a common trap is defaulting to subnet-based rules, which are less precise and harder to maintain. Remember the memory tip: "Tag it, then bag it"—assign a unique target tag to each tier, then write firewall rules that allow traffic only between the specific source and destination tags.
Google PCA Design for security and compliance Practice Question
This PCA practice question tests your understanding of design for security and compliance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A financial services company runs a multi-tier application on Compute Engine. They need to restrict network access so that only the web tier can communicate with the application tier, and only the application tier can access the database tier. All VMs are in the same VPC network. What is the most secure way to implement this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use VPC firewall rules with target tags to allow traffic between specific tiers.
VPC firewall rules with target tags allow you to precisely control ingress and egress traffic between VM instances based on their assigned tags. By tagging web tier VMs with a tag like 'web-tier' and application tier VMs with 'app-tier', you can create a firewall rule that allows traffic from 'web-tier' to 'app-tier' on the required port (e.g., TCP 8080) and another rule allowing traffic from 'app-tier' to 'db-tier' on the database port (e.g., TCP 3306). This approach enforces the principle of least privilege within a single VPC network without introducing unnecessary complexity or breaking network isolation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use Identity-Aware Proxy (IAP) to manage network access between tiers.
Why it's wrong here
IAP is for user access to instances, not for inter-tier traffic control.
✓
Use VPC firewall rules with target tags to allow traffic between specific tiers.
Why this is correct
VPC firewall rules with tags are the simplest and most secure way to enforce network segmentation within a VPC.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create separate VPC networks for each tier and use VPC peering.
Why it's wrong here
This adds complexity and is unnecessary; VPC firewall rules within a single network can achieve the goal.
✗
Assign a unique service account to each tier and use IAM conditions to restrict traffic.
Why it's wrong here
Service accounts are used for authentication, not network-level traffic control.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse IAM conditions or service accounts with network-layer access control, or they overcomplicate the solution by suggesting separate VPC networks when the simplest and most secure method within a single VPC is using firewall rules with target tags.
Detailed technical explanation
How to think about this question
VPC firewall rules are stateful by default, meaning that if you allow inbound traffic from the web tier to the application tier, the return traffic is automatically permitted without an explicit egress rule. Target tags are applied to VM instances via the network interface, and firewall rules evaluate these tags at the instance level, not the subnet level, allowing granular control even within the same subnet. In a real-world scenario, you might combine target tags with service accounts for defense in depth, but for pure network access control, firewall rules with tags are the correct and most secure method.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this PCA question in full detail.
Design for security and compliance — This question tests Design for security and compliance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use VPC firewall rules with target tags to allow traffic between specific tiers. — VPC firewall rules with target tags allow you to precisely control ingress and egress traffic between VM instances based on their assigned tags. By tagging web tier VMs with a tag like 'web-tier' and application tier VMs with 'app-tier', you can create a firewall rule that allows traffic from 'web-tier' to 'app-tier' on the required port (e.g., TCP 8080) and another rule allowing traffic from 'app-tier' to 'db-tier' on the database port (e.g., TCP 3306). This approach enforces the principle of least privilege within a single VPC network without introducing unnecessary complexity or breaking network isolation.
What should I do if I get this PCA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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