The answer is that a default firewall rule allowing HTTP exists with a higher priority. In Google Cloud Platform, firewall rules are evaluated strictly by their priority number, where a lower number means higher precedence, so the default-allow-http rule at priority 1000 is evaluated before a custom deny-all rule at priority 2000, allowing HTTP traffic to reach instances despite the deny rule. This scenario is a classic trap on the Google Professional Cloud Network Engineer exam, testing your understanding that GCP firewall rule priority order is numerical and that default rules (priority 65535 for implied, but 1000 for common allow rules) can override broader deny rules if their priority is lower. A common memory tip is to think of priority numbers like a race: the smallest number wins the evaluation race first, so always check the priority values, not just the rule action, when troubleshooting unexpected traffic.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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. 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.
Network Topology
Refer to the exhibit. Users report that HTTP (port 80) traffic is still reaching instances in my-vpc despite the deny-all rule. What is the most likely reason?
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
✓
A default firewall rule allowing HTTP exists with a higher priority.
In Google Cloud Platform (GCP), firewall rules are evaluated in order of priority, with lower numbers having higher priority. The default-allow-http rule has a priority of 1000, which is higher (lower number) than the deny-all rule's priority of 2000. Therefore, HTTP traffic is allowed by the default rule before the deny rule is evaluated, causing HTTP traffic to still reach instances.
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 allow-ssh rule has priority 1000 and is evaluated before the deny rule.
Why it's wrong here
The allow-ssh rule only allows SSH, not HTTP, so it does not affect HTTP traffic.
✓
A default firewall rule allowing HTTP exists with a higher priority.
Why this is correct
The default-allow-http rule (priority 1000) allows HTTP before the deny-all (priority 2000) is evaluated.
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 deny-all rule does not specify target tags.
Why it's wrong here
Without target tags, the rule applies to all instances, so it should block HTTP.
✗
The deny-all rule has priority 2000, which is higher than the allow rule.
Why it's wrong here
Priority 2000 is lower (higher number) than 1000, so it is evaluated after.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that higher priority numbers mean higher precedence, but in GCP firewall rules, lower priority numbers are evaluated first, so a deny rule with a higher priority number (e.g., 2000) is actually evaluated after an allow rule with a lower priority number (e.g., 1000).
Detailed technical explanation
How to think about this question
GCP firewall rules are stateful and evaluated in priority order (1-65535), where lower numbers are evaluated first. The default VPC includes implied allow rules for egress and ingress, but also pre-populated default rules like default-allow-http (priority 1000) and default-allow-https (priority 1000). When a custom deny-all rule is created with priority 2000, it is evaluated after these default allow rules, so HTTP traffic is permitted before the deny rule is reached. This behavior is distinct from AWS security groups, which are evaluated as a whole, and from Cisco ACLs, which use a sequential top-down evaluation.
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 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.
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: A default firewall rule allowing HTTP exists with a higher priority. — In Google Cloud Platform (GCP), firewall rules are evaluated in order of priority, with lower numbers having higher priority. The default-allow-http rule has a priority of 1000, which is higher (lower number) than the deny-all rule's priority of 2000. Therefore, HTTP traffic is allowed by the default rule before the deny rule is evaluated, causing HTTP traffic to still reach instances.
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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Question Discussion
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