PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring 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 security engineer is troubleshooting connectivity issues between two Compute Engine instances in the same VPC but in different subnets. Both instances have internal IPs and are in the same region. The firewall rules allow ingress from 10.0.0.0/8. However, traffic is failing. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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 ingress firewall rule is applied to a network tag that is not assigned to the destination instances.
Option C is correct because firewall rules in Google Cloud VPC are applied to the destination instance based on network tags, not just the subnet or IP range. If the ingress rule allowing traffic from 10.0.0.0/8 is configured with a target tag that is not assigned to the destination Compute Engine instances, the rule will not apply, and traffic will be dropped. This is a common misconfiguration when using tags to selectively apply firewall rules.
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 instances are using external IPs and the source IP is being NATed.
The VPC has dynamic routing mode set to global, causing routing conflicts.
Why it's wrong here
Global routing mode does not affect intra-region traffic.
✓
The ingress firewall rule is applied to a network tag that is not assigned to the destination instances.
Why this is correct
Firewall rules are applied to instances via tags or service accounts; missing tag would block traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
There is a firewall rule with a lower priority that denies egress traffic between subnets.
Why it's wrong here
By default egress is allowed; a deny rule would need to be explicitly created.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules applied to a subnet or IP range automatically apply to all instances in that subnet, when in reality, network tags are required to target specific instances unless the rule is applied to all instances (target = 'all instances').
Detailed technical explanation
How to think about this question
Google Cloud VPC firewall rules are stateful and can be applied to all instances in a VPC, or selectively using target tags or service accounts. When a rule uses a target tag, only instances with that tag are affected; if the destination instance lacks the tag, the rule is ignored, and the implicit deny-all ingress rule (priority 65535) blocks the traffic. This is distinct from AWS security groups, which are instance-level, or Azure NSGs, which are subnet- or NIC-level.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
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: The ingress firewall rule is applied to a network tag that is not assigned to the destination instances. — Option C is correct because firewall rules in Google Cloud VPC are applied to the destination instance based on network tags, not just the subnet or IP range. If the ingress rule allowing traffic from 10.0.0.0/8 is configured with a target tag that is not assigned to the destination Compute Engine instances, the rule will not apply, and traffic will be dropped. This is a common misconfiguration when using tags to selectively apply firewall rules.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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