- A
Remove the default allow egress rule that allows all outbound traffic
Why wrong: Removing the default egress rule would block all outbound traffic, which could break other communications. The specific egress rule will take precedence due to higher priority if needed, but actually the default egress rule allows all, and our specific rule allows additional traffic; it doesn't conflict. We don't need to remove it.
- B
Create an egress firewall rule that allows TCP 4444 from target tag 'app-tier' to source tag 'app-tier'
This rule allows outbound traffic on port 4444 from instances with the tag 'app-tier' to instances with the same tag.
- C
Create a VPC firewall rule to deny all egress traffic from the VPC
Why wrong: This would block all outbound traffic, including necessary communications. The default allow egress rule can remain; we just need to ensure specific allow rules for the required traffic.
- D
Create an ingress firewall rule that allows TCP 4444 from source tag 'app-tier' to target tag 'app-tier'
This rule allows inbound traffic on port 4444 between instances with the tag 'app-tier'.
- E
Apply the 'app-tier' tag to all instances that need to communicate
Tags are used to select instances for firewall rules. Without the tag, the rules won't apply.
Firewall Rules with Tags for Restricting Inter-Instance Traffic
This PCSE practice question tests your understanding of your company has a vpc with multiple subnets. 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.
Your company has a VPC with multiple subnets. You have deployed a set of Compute Engine instances that must communicate with each other over TCP port 4444. The instances are tagged with 'app-tier'. You need to ensure that only these instances can communicate on this port. Which THREE of the following steps are necessary to achieve this?
Quick Answer
The answer is to apply the 'app-tier' tag to all instances that need to communicate, then create an egress firewall rule allowing TCP 4444 with the source tag set to 'app-tier' and the destination tag set to 'app-tier'. This works because firewall rules with tags restrict inter-instance communication by evaluating the tags on the source and destination instances, ensuring that only tagged instances can send or receive traffic on that port. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding of how VPC firewall rules leverage network tags for micro-segmentation, a common trap being that candidates mistakenly apply an ingress rule instead of the egress rule, which would not control which instances can initiate the outbound connection. Remember that egress rules govern who can send traffic, while ingress rules govern who can receive it; a useful memory tip is "egress enforces the source, ingress inspects the destination."
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 egress firewall rule that allows TCP 4444 from target tag 'app-tier' to source tag 'app-tier'
To restrict TCP port 4444 communication to only instances tagged 'app-tier', three steps are necessary. First, apply the 'app-tier' tag to all relevant instances (option E) so that firewall rules can identify them. Second, create an ingress firewall rule allowing TCP 4444 from source tag 'app-tier' to target tag 'app-tier' (option D); this ensures that only 'app-tier' instances can receive traffic on port 4444. Third, create an egress firewall rule allowing TCP 4444 from source tag 'app-tier' to destination tag 'app-tier' (option B); although the default egress rule allows all outbound traffic, this more specific rule ensures that only 'app-tier' instances can send traffic on port 4444 to other 'app-tier' instances. Note that the default egress rule is an allow-all, not deny; the combination of ingress and egress rules with tags tightly controls the allowed communication.
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.
- ✗
Remove the default allow egress rule that allows all outbound traffic
Why it's wrong here
Removing the default egress rule would block all outbound traffic, which could break other communications. The specific egress rule will take precedence due to higher priority if needed, but actually the default egress rule allows all, and our specific rule allows additional traffic; it doesn't conflict. We don't need to remove it.
- ✓
Create an egress firewall rule that allows TCP 4444 from target tag 'app-tier' to source tag 'app-tier'
Why this is correct
This rule allows outbound traffic on port 4444 from instances with the tag 'app-tier' to instances with the same tag.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create a VPC firewall rule to deny all egress traffic from the VPC
Why it's wrong here
This would block all outbound traffic, including necessary communications. The default allow egress rule can remain; we just need to ensure specific allow rules for the required traffic.
- ✓
Create an ingress firewall rule that allows TCP 4444 from source tag 'app-tier' to target tag 'app-tier'
Why this is correct
This rule allows inbound traffic on port 4444 between instances with the tag 'app-tier'.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Apply the 'app-tier' tag to all instances that need to communicate
Why this is correct
Tags are used to select instances for firewall rules. Without the tag, the rules won't apply.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common mistake is assuming that only ingress rules are necessary because GCP firewall rules are stateful. While stateful rules automatically allow return traffic, they do not restrict which instances can initiate outbound connections. Since the default egress rule is allow all, you must also create an egress rule to limit which sources can send traffic on the port. Additionally, candidates often forget to apply the 'app-tier' tag to the instances, which is required for the firewall rules to match.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules are stateful and evaluated in order of priority (lower number = higher priority). The default egress rule (priority 65535) allows all outbound traffic, but a higher-priority rule (e.g., priority 1000) can deny or allow specific traffic. The use of target tags and source tags allows granular control based on instance metadata, enabling micro-segmentation without relying on IP addresses. This is similar to security group rules in AWS but with explicit priority ordering.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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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 egress firewall rule that allows TCP 4444 from target tag 'app-tier' to source tag 'app-tier' — To restrict TCP port 4444 communication to only instances tagged 'app-tier', three steps are necessary. First, apply the 'app-tier' tag to all relevant instances (option E) so that firewall rules can identify them. Second, create an ingress firewall rule allowing TCP 4444 from source tag 'app-tier' to target tag 'app-tier' (option D); this ensures that only 'app-tier' instances can receive traffic on port 4444. Third, create an egress firewall rule allowing TCP 4444 from source tag 'app-tier' to destination tag 'app-tier' (option B); although the default egress rule allows all outbound traffic, this more specific rule ensures that only 'app-tier' instances can send traffic on port 4444 to other 'app-tier' instances. Note that the default egress rule is an allow-all, not deny; the combination of ingress and egress rules with tags tightly controls the allowed communication.
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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