This PCSE practice question tests your understanding of pcse exam topics. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Refer to the exhibit. A VM in the default network with internal IP 10.128.0.5 is unable to reach a VM at 10.0.0.4 over TCP port 22. 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.
The deny-all rule blocks traffic from 10.0.0.4 to the VM.
Why wrong: The deny-all rule denies traffic from 10.0.0.0/8, which includes 10.0.0.4, so it does block traffic from that IP, but the question asks for the cause; the priority override is the key.
B
The deny-all rule has higher priority than default-allow-internal.
Deny-all (priority 2000) has higher priority (numerically lower) than default-allow-internal (priority 65534), so it overrides the allow rule.
C
The default-allow-internal rule does not cover port 22.
Why wrong: Default-allow-internal allows all TCP ports, including 22.
D
There is no rule allowing SSH traffic from 10.0.0.4.
Why wrong: Default-allow-internal allows all TCP, so SSH is allowed; but it is overridden by the deny rule.
The answer is a VPC firewall rule priority conflict blocking SSH, specifically because the deny-all rule with priority 2000 takes precedence over the default-allow-internal rule with priority 65534. In Google Cloud VPC firewall rules, a lower priority number means higher precedence, so the deny-all rule, which blocks all TCP traffic from the 10.0.0.0/8 range, overrides the permissive default-allow-internal rule and prevents SSH (TCP port 22) from reaching the VM at 10.128.0.5. This scenario tests your understanding of firewall rule evaluation order and priority hierarchies, a common trap on the Google Professional Cloud Security Engineer exam where candidates mistakenly assume that a more specific allow rule always wins. The key insight is that explicit deny rules with higher priority (lower number) will always block traffic, even if a lower-priority allow rule exists. Memory tip: think of priority numbers like a race—lower numbers finish first and get the final say.
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
✓
The deny-all rule has higher priority than default-allow-internal.
Option B is correct. The deny-all rule (priority 2000) has a higher priority (lower number) than the default-allow-internal rule (priority 65534). Since the deny-all rule blocks all TCP traffic from the 10.0.0.0/8 range, it overrides the default-allow-internal rule, blocking SSH traffic from 10.0.0.4 to 10.128.0.5. Option A is incorrect because the deny rule blocks traffic from 10.0.0.0/8, which includes 10.0.0.4, so it does block traffic, but the most likely cause is the priority ordering. Option C is incorrect because the default-allow-internal rule covers all TCP ports, including port 22. Option D is incorrect because there is a rule (default-allow-internal) that allows SSH traffic, but the deny-all rule takes precedence due to higher priority.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 deny-all rule blocks traffic from 10.0.0.4 to the VM.
Why it's wrong here
The deny-all rule denies traffic from 10.0.0.0/8, which includes 10.0.0.4, so it does block traffic from that IP, but the question asks for the cause; the priority override is the key.
✓
The deny-all rule has higher priority than default-allow-internal.
Why this is correct
Deny-all (priority 2000) has higher priority (numerically lower) than default-allow-internal (priority 65534), so it overrides the allow rule.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The default-allow-internal rule does not cover port 22.
Why it's wrong here
Default-allow-internal allows all TCP ports, including 22.
✗
There is no rule allowing SSH traffic from 10.0.0.4.
Why it's wrong here
Default-allow-internal allows all TCP, so SSH is allowed; but it is overridden by the deny rule.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCSE ACL questions on filtering logic and placement.
The correct answer is: The deny-all rule has higher priority than default-allow-internal. — Option B is correct. The deny-all rule (priority 2000) has a higher priority (lower number) than the default-allow-internal rule (priority 65534). Since the deny-all rule blocks all TCP traffic from the 10.0.0.0/8 range, it overrides the default-allow-internal rule, blocking SSH traffic from 10.0.0.4 to 10.128.0.5. Option A is incorrect because the deny rule blocks traffic from 10.0.0.0/8, which includes 10.0.0.4, so it does block traffic, but the most likely cause is the priority ordering. Option C is incorrect because the default-allow-internal rule covers all TCP ports, including port 22. Option D is incorrect because there is a rule (default-allow-internal) that allows SSH traffic, but the deny-all rule takes precedence due to higher priority.
What should I do if I get this PCSE question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related PCSE ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which THREE steps are most effective for troubleshooting a VPC firewall rule issue where desired traffic is being blocked?
hard
A.Check IAM permissions for the VM's service account
✓ B.Check firewall rule logs in Cloud Logging
✓ C.Verify that the network tags on instances match the target tags in the firewall rule
✓ D.Review VPC flow logs to see if traffic is reaching the VM
E.Inspect the route tables for the subnet
Why B: Options B, C, and D are correct. Checking firewall rule logs in Cloud Logging (B) reveals whether traffic was denied by a firewall rule. Verifying that network tags on instances match the target tags in the firewall rule (C) ensures the rule applies to the correct VMs. Reviewing VPC flow logs (D) shows actual traffic flows to confirm whether traffic reaches the VM. Option A (check IAM permissions) is less relevant because firewall rules are network-level, not identity-based. Option E (inspect route tables) affects routing, not firewall blocking.
Variation 2. An engineer notices that traffic on port 80 is not reaching instances with the tag 'http-server'. The instances have external IPs and are in the default VPC. What could be the reason?
easy
A.The sourceRanges does not include the instance's public IP.
B.The targetTags are not applied to the instances.
✓ C.A deny rule with a lower priority number (higher priority) exists and blocks the traffic.
D.The direction should be EGRESS.
Why C: Option C is correct because a deny rule with a lower priority number (higher priority) will override the allow rule for port 80, blocking traffic. Option A is incorrect because sourceRanges refers to allowed source IPs, not the instance's public IP. Option B is incorrect because if the targetTags are not applied, the rule would not affect those instances, but the issue is specifically about traffic not reaching instances with the tag 'http-server', implying the allow rule exists but is overridden. Option D is incorrect because direction should be INGRESS for incoming traffic, not EGRESS.
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