The answer is to check VPC firewall rules to ensure ingress traffic from the on-premises subnet is allowed to the VM. This is correct because even when a VPN tunnel is established, Google Cloud’s default-deny ingress posture means that all incoming traffic, including traffic arriving through the tunnel, is blocked unless explicitly permitted by a firewall rule. The tunnel being up only confirms the control plane is working; the data plane still requires a rule allowing the on-premises source CIDR and the VM’s target tag or service account. On the Google Professional Cloud Network Engineer exam, this question tests your understanding that firewall rules are the first line of defense for VM traffic, and a common trap is to assume an established tunnel guarantees connectivity. A useful memory tip is “tunnel up does not mean traffic through”—always verify the firewall rule before checking routes or BGP.
PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing 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 network engineer is troubleshooting connectivity from an on-premises network to a GCE VM through a VPN tunnel. The tunnel is established, but traffic is not reaching the VM. What should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check VPC firewall rules to ensure ingress traffic from the on-premises subnet is allowed to the VM
Option A is correct because VPC firewall rules are the first line of defense for controlling traffic to GCE VMs. Even though the VPN tunnel is established, the default-deny ingress posture of GCP means that traffic from the on-premises subnet must be explicitly allowed by a VPC firewall rule. Without this rule, packets arriving via the tunnel are dropped before they ever reach the VM's network interface.
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.
✓
Check VPC firewall rules to ensure ingress traffic from the on-premises subnet is allowed to the VM
Why this is correct
Firewall rules must allow traffic from the on-premises IP range to the VM's target tags or service account.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Check the VM's OS firewall to see if it is blocking incoming traffic
Why it's wrong here
While possible, the VPC firewall is the first layer of defense and easier to check.
✗
Verify that the VPN tunnel is using the correct pre-shared key
Why it's wrong here
If the tunnel is established, the PSK is correct.
✗
Review Cloud Armor security policies that may be blocking the traffic
Why it's wrong here
Cloud Armor applies to load-balanced traffic, not direct VPN traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a working tunnel implies all traffic flows, but the trap here is that VPC firewall rules are independent of tunnel status and must be explicitly configured for the on-premises subnet as the source.
Detailed technical explanation
How to think about this question
GCP VPC firewall rules are stateful and evaluated at the subnet level, not at the VM. For VPN traffic, the source IP is the on-premises subnet's CIDR, and the destination is the VM's internal IP. The rule must allow ingress from that CIDR to the VM's target tag or service account. A common subtlety is that the firewall rule must reference the correct network tag or service account; otherwise, even a seemingly correct rule will not apply.
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.
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: Check VPC firewall rules to ensure ingress traffic from the on-premises subnet is allowed to the VM — Option A is correct because VPC firewall rules are the first line of defense for controlling traffic to GCE VMs. Even though the VPN tunnel is established, the default-deny ingress posture of GCP means that traffic from the on-premises subnet must be explicitly allowed by a VPC firewall rule. Without this rule, packets arriving via the tunnel are dropped before they ever reach the VM's network interface.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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