The answer is a firewall rule with priority 500 that denies ingress traffic to the specific VM’s tag or IP range. In Google Cloud, VPC firewall rule priority order dictates that rules are evaluated from the lowest numerical priority value to the highest, so a deny rule at priority 500 overrides an allow rule at priority 1000, even if the allow rule matches the same tag. This explains why other VMs with the ‘ssh-allowed’ tag remain reachable—they are not caught by the specific deny rule’s target. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that lower priority numbers mean higher precedence, and a common trap is assuming all VMs with the same tag behave identically. Remember the mnemonic: “Low number wins the fight”—the rule with the smallest priority number always takes effect first, whether it allows or denies.
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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.
Exhibit
Refer to the exhibit.
gcloud compute firewall-rules describe allow-ssh
Allowed:
IPProtocol: tcp
ports: ['22']
Direction: INGRESS
Source ranges: ['0.0.0.0/0']
Target tags: ['ssh-allowed']
Priority: 1000
A VM with network tag 'ssh-allowed' cannot be reached via SSH from the internet. Other VMs with the same tag work fine.
Refer to the exhibit. A VM with the 'ssh-allowed' tag is unreachable via SSH from the internet, while other VMs with the same tag work. 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.
Refer to the exhibit.
gcloud compute firewall-rules describe allow-ssh
Allowed:
IPProtocol: tcp
ports: ['22']
Direction: INGRESS
Source ranges: ['0.0.0.0/0']
Target tags: ['ssh-allowed']
Priority: 1000
A VM with network tag 'ssh-allowed' cannot be reached via SSH from the internet. Other VMs with the same tag work fine.
A
A firewall rule with priority 500 denies ingress traffic to the VM's tag or IP range.
A higher priority deny rule can override the allow rule.
B
The rule source range is set to 0.0.0.0/0, which includes all internet IPs, so it should allow SSH.
Why wrong: That doesn't explain why only one VM fails.
C
The VM is in a different VPC that does not have the allow-ssh rule.
Why wrong: If the VM is in the same VPC, the rule applies.
D
The firewall rule 'allow-ssh' has a higher priority (1000) than the implicit deny (65535), so it should work.
Why wrong: That is correct, but doesn't explain the issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A firewall rule with priority 500 denies ingress traffic to the VM's tag or IP range.
The most likely cause is that a firewall rule with priority 500 explicitly denies ingress traffic to the specific VM's tag or IP range, overriding the allow-ssh rule (which has a lower priority, i.e., a higher numerical value). In Google Cloud Platform (GCP), firewall rules are evaluated from lowest to highest priority number, and a deny rule with a lower priority number (e.g., 500) takes precedence over an allow rule with a higher priority number (e.g., 1000). This explains why other VMs with the same 'ssh-allowed' tag remain reachable, as they are not affected by the specific deny rule.
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.
✓
A firewall rule with priority 500 denies ingress traffic to the VM's tag or IP range.
Why this is correct
A higher priority deny rule can override the allow rule.
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.
✗
The rule source range is set to 0.0.0.0/0, which includes all internet IPs, so it should allow SSH.
The firewall rule 'allow-ssh' has a higher priority (1000) than the implicit deny (65535), so it should work.
Why it's wrong here
That is correct, but doesn't explain the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a higher priority number means higher priority, when in fact a lower priority number (e.g., 500) takes precedence over a higher one (e.g., 1000), causing candidates to overlook the effect of a deny rule with a lower priority number.
Detailed technical explanation
How to think about this question
In GCP, firewall rules are evaluated based on priority, where a lower numerical value indicates higher priority. The implicit deny rule at priority 65535 is the lowest priority, but explicit deny rules with lower priority numbers (e.g., 500) can override allow rules (e.g., priority 1000). This hierarchical evaluation ensures that specific deny rules can block traffic even if a broader allow rule exists. In practice, this is commonly used to create exceptions, such as blocking SSH from specific IP ranges while allowing it from others, which can lead to unexpected behavior if not carefully managed.
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 services — This question tests Configuring network services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A firewall rule with priority 500 denies ingress traffic to the VM's tag or IP range. — The most likely cause is that a firewall rule with priority 500 explicitly denies ingress traffic to the specific VM's tag or IP range, overriding the allow-ssh rule (which has a lower priority, i.e., a higher numerical value). In Google Cloud Platform (GCP), firewall rules are evaluated from lowest to highest priority number, and a deny rule with a lower priority number (e.g., 500) takes precedence over an allow rule with a higher priority number (e.g., 1000). This explains why other VMs with the same 'ssh-allowed' tag remain reachable, as they are not affected by the specific deny rule.
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: "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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