- A
Create an ingress rule allowing traffic from the web tier subnet to the application tier subnet, and an ingress rule allowing traffic from the application tier subnet to database tier subnet.
Why wrong: Subnet-based rules are less granular; if instances are in different subnets, this works, but it allows any instance in that subnet to access the other tier, not just the intended tier.
- B
Assign each tier a unique service account and create ingress rules allowing traffic from the appropriate service accounts.
Why wrong: Service accounts can be used for firewall rules, but they are typically used for outbound traffic; egress rules can use source service accounts, but ingress rules use source IPs not service accounts.
- C
Create an ingress rule allowing traffic from 0.0.0.0/0 to instances with tag 'web', an ingress rule allowing traffic from instances with tag 'web' to instances with tag 'app', and an ingress rule allowing traffic from instances with tag 'app' to instances with tag 'db'.
Tags provide simple group-based access control.
- D
Create a single ingress rule that allows all traffic within the VPC network and a separate rule to allow internet traffic to web tier.
Why wrong: This would allow the application tier to directly access the database tier, which is not desired.
Segregate Web, App, and Database Tiers with VPC Firewall Tags
This PCSE practice question tests your understanding of designing a network for a multi-tier application. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are designing a network for a multi-tier application. The web tier must be accessible from the internet, while the application tier should only be accessible from the web tier. The database tier should only be accessible from the application tier. All tiers are in the same VPC. Which combination of firewall rules meets these requirements?
Quick Answer
The correct answer is to create ingress rules that allow traffic from 0.0.0.0/0 to instances tagged 'web', from 'web'-tagged instances to 'app'-tagged instances, and from 'app'-tagged instances to 'db'-tagged instances. This configuration enforces multi-tier segmentation with firewall tags by applying least-privilege access at each layer: the web tier is the only one exposed to the internet, the application tier is locked down to only accept traffic from the web tier, and the database tier is isolated to only accept traffic from the application tier. On the Google Professional Cloud Security Engineer exam, this scenario tests your ability to implement network segmentation within a single VPC using tag-based firewall rules, a common alternative to separate VPCs or subnets. A frequent trap is to allow all internal traffic or to open the database tier to the web tier directly, which violates the principle of defense in depth. Remember the chain: web opens to the world, app only from web, db only from app—think of it as a one-way street where each tier can only talk to the tier directly behind it.
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
Create an ingress rule allowing traffic from 0.0.0.0/0 to instances with tag 'web', an ingress rule allowing traffic from instances with tag 'web' to instances with tag 'app', and an ingress rule allowing traffic from instances with tag 'app' to instances with tag 'db'.
Option C is correct because it uses VPC firewall tags to enforce least-privilege network segmentation: the web tier is exposed to the internet (0.0.0.0/0), the app tier only accepts traffic from web-tagged instances, and the database tier only accepts traffic from app-tagged instances. This matches the multi-tier access requirements without exposing internal tiers to the internet or to each other unnecessarily. Option B is incorrect because while firewall rules in Google Cloud support source service accounts, they cannot be used to allow traffic from the internet, as internet traffic does not have a service account. The web tier's internet access requirement breaks the service account approach.
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.
- ✗
Create an ingress rule allowing traffic from the web tier subnet to the application tier subnet, and an ingress rule allowing traffic from the application tier subnet to database tier subnet.
Why it's wrong here
Subnet-based rules are less granular; if instances are in different subnets, this works, but it allows any instance in that subnet to access the other tier, not just the intended tier.
- ✗
Assign each tier a unique service account and create ingress rules allowing traffic from the appropriate service accounts.
- ✓
Create an ingress rule allowing traffic from 0.0.0.0/0 to instances with tag 'web', an ingress rule allowing traffic from instances with tag 'web' to instances with tag 'app', and an ingress rule allowing traffic from instances with tag 'app' to instances with tag 'db'.
Why this is correct
Tags provide simple group-based access control.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a single ingress rule that allows all traffic within the VPC network and a separate rule to allow internet traffic to web tier.
Why it's wrong here
This would allow the application tier to directly access the database tier, which is not desired.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is assuming that because firewall rules can use service accounts for internal segmentation, they can also handle internet ingress. However, internet traffic has no service account, so tags (or IP ranges) are required for 0.0.0.0/0 rules.
Detailed technical explanation
How to think about this question
VPC firewall rules are stateful by default, meaning that if you allow inbound traffic from a source, the corresponding outbound return traffic is automatically permitted. When using target tags, the rule applies only to instances that have that tag, and the source filter can be another tag (e.g., source tag 'web' matches instances with that tag, using their internal IPs). This tag-based approach scales well in dynamic environments where instance IPs change, and it avoids the need to manage subnet CIDR ranges.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this PCSE question test?
Read the scenario before looking for a memorised answer.
What is the correct answer to this question?
The correct answer is: Create an ingress rule allowing traffic from 0.0.0.0/0 to instances with tag 'web', an ingress rule allowing traffic from instances with tag 'web' to instances with tag 'app', and an ingress rule allowing traffic from instances with tag 'app' to instances with tag 'db'. — Option C is correct because it uses VPC firewall tags to enforce least-privilege network segmentation: the web tier is exposed to the internet (0.0.0.0/0), the app tier only accepts traffic from web-tagged instances, and the database tier only accepts traffic from app-tagged instances. This matches the multi-tier access requirements without exposing internal tiers to the internet or to each other unnecessarily. Option B is incorrect because while firewall rules in Google Cloud support source service accounts, they cannot be used to allow traffic from the internet, as internet traffic does not have a service account. The web tier's internet access requirement breaks the service account approach.
What should I do if I get this PCSE 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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Last reviewed: Jun 30, 2026
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