- A
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
Correct. This ingress rule allows TCP 3306 from source tag 'app' to target tag 'db', permitting the desired connections.
- B
Egress rule: target tag 'app', destination tag 'db', allow tcp:3306, priority 1000.
Why wrong: Incorrect. An egress rule on 'app' instances would not limit inbound connections to 'db' instances; it controls outbound traffic from 'app'.
- C
Ingress rule: target tag 'db', source 0.0.0.0/0, deny tcp:3306, priority 2000.
Correct. This ingress rule denies TCP 3306 from all sources (except the allowed app tag) with a lower priority than the allow rule, effectively blocking unauthorized traffic.
- D
Ingress rule: target tag 'db', source tag 'app', deny tcp:3306, priority 1000.
Why wrong: Incorrect. This rule would deny TCP 3306 from app tag, which contradicts the requirement.
- E
Ingress rule: target tag 'db', source 0.0.0.0/0, deny all ports, priority 500.
Why wrong: Incorrect. Denying all ports from all sources with too high a priority (lower number) would block all traffic, including the allowed connections.
PCNE Firewall rule priority Practice Question
This PCNE practice question tests your understanding of implementing vpc instances. 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. A key principle to apply: firewall rule priority. 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 need to allow instances with network tag 'db' in subnet-a to only accept connections on TCP port 3306 from instances with network tag 'app' in subnet-b. Which TWO firewall rules should you create? (Choose 2)
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
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
To allow only instances with tag 'app' in subnet-b to connect to instances with tag 'db' in subnet-a on TCP port 3306, you need two ingress firewall rules on the db instances: one to allow traffic from the app source tag, and one to deny traffic from all other sources. The allow rule must have a higher priority (lower number) than the deny rule to take effect. Thus, option A (allow from app, priority 1000) and option C (deny from all, priority 2000) together achieve the goal.
Key principle: Firewall rule priority
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000.
- ✗
Egress rule: target tag 'app', destination tag 'db', allow tcp:3306, priority 1000.
Why it's wrong here
Incorrect. An egress rule on 'app' instances would not limit inbound connections to 'db' instances; it controls outbound traffic from 'app'.
- ✓
Ingress rule: target tag 'db', source 0.0.0.0/0, deny tcp:3306, priority 2000.
- ✗
Ingress rule: target tag 'db', source tag 'app', deny tcp:3306, priority 1000.
Why it's wrong here
Incorrect. This rule would deny TCP 3306 from app tag, which contradicts the requirement.
- ✗
Ingress rule: target tag 'db', source 0.0.0.0/0, deny all ports, priority 500.
Why it's wrong here
Incorrect. Denying all ports from all sources with too high a priority (lower number) would block all traffic, including the allowed connections.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Firewall rule priority
- Ingress rule
- Source tags
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
Firewall rule priority
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
Review firewall rule priority, then practise related PCNE questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this PCNE question test?
Implementing VPC Instances — This question tests Implementing VPC Instances — Firewall rule priority.
What is the correct answer to this question?
The correct answer is: Ingress rule: target tag 'db', source tag 'app', allow tcp:3306, priority 1000. — To allow only instances with tag 'app' in subnet-b to connect to instances with tag 'db' in subnet-a on TCP port 3306, you need two ingress firewall rules on the db instances: one to allow traffic from the app source tag, and one to deny traffic from all other sources. The allow rule must have a higher priority (lower number) than the deny rule to take effect. Thus, option A (allow from app, priority 1000) and option C (deny from all, priority 2000) together achieve the goal.
What should I do if I get this PCNE question wrong?
Review firewall rule priority, then practise related PCNE questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Firewall rule priority
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Last reviewed: Jul 4, 2026
This PCNE 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 PCNE exam.
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