The answer is that the app instances need a firewall rule to allow egress traffic to the database on port 3306. This is correct because Google Cloud VPC firewall rules are stateful for ingress but not for egress; the ingress rule you created only controls incoming packets to the database instance, while the outbound SYN packet from the app instance is dropped without a corresponding egress rule. Deleting the default allow internal rule (priority 65535) removed the implicit egress permission that previously allowed all outbound traffic, so a specific egress rule is now required to permit the connection. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of VPC firewall egress rule requirements and the common trap of assuming ingress rules alone handle bidirectional traffic. A helpful memory tip is “ingress opens the door, egress lets you walk through it”—always verify both directions when default rules are removed.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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.
A company has a single VPC with subnets in us-central1 and europe-west1. They have Compute Engine instances in both subnets that need to communicate with each other. The security team wants to ensure that only specific instances in us-central1 can connect to a database instance in europe-west1 on port 3306. Currently, the default firewall rules allow all internal traffic (priority 65535). The network engineer first creates a new ingress firewall rule to allow TCP traffic on port 3306 from instances with the network tag 'app' to instances with the tag 'db', with priority 1000. Then, to enforce the restriction, they delete the default allow internal rule (priority 65535). However, after applying the changes, the app instances (tagged 'app') in us-central1 cannot connect to the database instance (tagged 'db') in europe-west1. The engineer verifies that the tags are correctly applied to the instances. What is the most likely cause of the connectivity failure?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The app instances need a firewall rule to allow egress traffic to the database on port 3306.
B is correct because in Google Cloud VPC, firewall rules are stateful for ingress but not for egress. The ingress rule allowing traffic from 'app' to 'db' on port 3306 only controls incoming packets to the database instance. The app instance still needs an egress firewall rule to allow outbound traffic on port 3306, otherwise the outbound SYN packet is dropped before it reaches the database. Deleting the default allow internal rule (priority 65535) removed the implicit egress permission, so a specific egress rule is required.
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.
✗
The firewall rule only allows ingress from instances with tag 'app' but the egress traffic from app instances is blocked.
Why it's wrong here
While egress is blocked, the issue is specifically that no egress rule exists; this option is vague.
✓
The app instances need a firewall rule to allow egress traffic to the database on port 3306.
Why this is correct
With the default allow internal rule removed, egress must be explicitly allowed.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The firewall rule is applied to the wrong VPC network.
Why it's wrong here
The rule was applied to the correct VPC since it was created in that context.
✗
The database instance's network tag 'db' was not applied to the database instance.
Why it's wrong here
The engineer verified tags are applied correctly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that an ingress rule alone is sufficient for bidirectional communication, but in Google Cloud VPC, egress rules are required for outbound traffic initiation unless a default allow egress rule exists.
Detailed technical explanation
How to think about this question
Google Cloud VPC firewall rules are stateful for ingress connections, meaning that if an ingress rule allows a packet, the corresponding return traffic is automatically allowed. However, egress traffic must be explicitly permitted by an egress firewall rule unless a higher-priority rule (like the default allow internal rule) covers it. When the default allow internal rule (priority 65535) is deleted, all egress traffic from instances is blocked unless a specific egress rule is created. In this scenario, the app instances need an egress rule with direction=EGRESS, target tags='app', source tags='db', protocol=tcp, port=3306, and priority lower than 65535 to allow outbound connections to the database.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The app instances need a firewall rule to allow egress traffic to the database on port 3306. — B is correct because in Google Cloud VPC, firewall rules are stateful for ingress but not for egress. The ingress rule allowing traffic from 'app' to 'db' on port 3306 only controls incoming packets to the database instance. The app instance still needs an egress firewall rule to allow outbound traffic on port 3306, otherwise the outbound SYN packet is dropped before it reaches the database. Deleting the default allow internal rule (priority 65535) removed the implicit egress permission, so a specific egress rule is required.
What should I do if I get this PCNE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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