- A
Add an ingress firewall rule in VPC-B allowing traffic from 10.0.0.0/16 on port 443.
Ingress rule on the target network is required; default egress allows outbound from source.
- B
Add an ingress firewall rule in VPC-A allowing traffic from 172.16.0.0/16 on port 443.
Why wrong: Ingress rule on VPC-A would allow traffic from VPC-B to VPC-A, not the reverse.
- C
Add a single firewall rule in the project with source 10.0.0.0/16 and destination 172.16.0.0/16 on port 443.
Why wrong: Firewall rules apply to a single VPC network; cannot apply to both networks in one rule.
- D
Add an egress firewall rule in VPC-A allowing traffic to 172.16.0.0/16 on port 443.
Why wrong: Default egress rule already allows all outbound traffic, so no egress rule is needed unless you have a deny egress rule.
Quick Answer
The answer is to add an ingress firewall rule in VPC-B allowing traffic from 10.0.0.0/16 on TCP port 443. This is correct because VPC peering does not automatically permit traffic between peered networks; firewall rules must explicitly authorize the communication. Since the instance in VPC-A initiates the connection to VPC-B, only the destination VPC’s ingress needs to be opened—egress rules in VPC-A are implicitly permissive by default in Google Cloud, making this the minimal configuration. On the Google Professional Cloud Security Engineer exam, this scenario tests your understanding that VPC peering is a connectivity mechanism, not a security boundary, and that firewall rules are evaluated at the destination. A common trap is assuming both ingress and egress rules are needed, or that peering alone grants access. Remember the memory tip: “Peering opens the door, but firewalls decide who walks through—only the destination’s ingress matters for initiated traffic.”
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 company has two VPC networks in the same project: VPC-A (10.0.0.0/16) and VPC-B (172.16.0.0/16). They have established VPC peering between them. An instance in VPC-A needs to communicate with an instance in VPC-B on TCP port 443. What is the minimal firewall configuration needed?
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
Add an ingress firewall rule in VPC-B allowing traffic from 10.0.0.0/16 on port 443.
Option A is correct because VPC peering does not automatically allow traffic; firewall rules must explicitly permit the desired communication. Since the instance in VPC-A initiates the connection to VPC-B, VPC-B's firewall must have an ingress rule allowing traffic from VPC-A's CIDR (10.0.0.0/16) on TCP port 443. This is the minimal configuration because egress rules in VPC-A are implicitly permissive by default in Google Cloud, and only the destination VPC's ingress needs to be opened.
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.
- ✓
Add an ingress firewall rule in VPC-B allowing traffic from 10.0.0.0/16 on port 443.
Why this is correct
Ingress rule on the target network is required; default egress allows outbound from source.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add an ingress firewall rule in VPC-A allowing traffic from 172.16.0.0/16 on port 443.
Why it's wrong here
Ingress rule on VPC-A would allow traffic from VPC-B to VPC-A, not the reverse.
- ✗
Add a single firewall rule in the project with source 10.0.0.0/16 and destination 172.16.0.0/16 on port 443.
Why it's wrong here
Firewall rules apply to a single VPC network; cannot apply to both networks in one rule.
- ✗
Add an egress firewall rule in VPC-A allowing traffic to 172.16.0.0/16 on port 443.
Why it's wrong here
Default egress rule already allows all outbound traffic, so no egress rule is needed unless you have a deny egress rule.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that VPC peering automatically opens all traffic between the peered networks, leading candidates to think no firewall rules are needed, or that egress rules must be added on the source side.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, firewall rules are stateful, meaning that if an ingress rule allows incoming traffic, the corresponding return traffic is automatically permitted without an egress rule. However, the initial inbound connection must be explicitly allowed by an ingress rule on the destination network. VPC peering does not imply any firewall rule changes; it only establishes routing between the networks. A common real-world scenario is when an application in VPC-A needs to reach a database in VPC-B on a specific port, and only the database's VPC needs an ingress rule for the application's CIDR.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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?
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add an ingress firewall rule in VPC-B allowing traffic from 10.0.0.0/16 on port 443. — Option A is correct because VPC peering does not automatically allow traffic; firewall rules must explicitly permit the desired communication. Since the instance in VPC-A initiates the connection to VPC-B, VPC-B's firewall must have an ingress rule allowing traffic from VPC-A's CIDR (10.0.0.0/16) on TCP port 443. This is the minimal configuration because egress rules in VPC-A are implicitly permissive by default in Google Cloud, and only the destination VPC's ingress needs to be opened.
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
This PCSE 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 PCSE exam.
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