The answer is that the VPC firewall rules blocking ingress traffic from the on-premises network are the most likely cause. Even when a Cloud VPN tunnel is established and the connection appears active, the default VPC network includes firewall rules that permit outbound traffic but deny all inbound traffic from external sources, including the on-premises server’s IP range. This means that while the tunnel itself is up, any ICMP packets (like ping) or other traffic from the on-premises side are dropped at the VPC firewall ingress, preventing the Compute Engine instance from receiving a response. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that VPN connectivity is a network-layer path, but application-layer communication still requires explicit firewall rules—a common trap is assuming the tunnel alone guarantees traffic flow. Remember the key distinction: a VPN tunnel solves routing, not firewall filtering. Memory tip: “Tunnel up, traffic stuck? Check the ingress luck.”
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 developer created a Compute Engine instance in the default VPC network. The instance needs to communicate with an on-premises server over a Cloud VPN tunnel. The developer configured the VPN tunnel but the instances cannot ping the on-premises server. 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 firewall rules in the VPC are blocking ingress traffic from the on-premises network.
Option D is correct because, by default, the default VPC includes firewall rules that allow outbound traffic but restrict inbound traffic. Even though the Cloud VPN tunnel is established, the VPC firewall rules block ingress traffic from the on-premises network (e.g., ICMP for ping). To allow communication, a firewall rule must explicitly permit ingress traffic from the on-premises IP range or the remote tunnel IP.
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 subnet IP range overlaps with the on-premises network.
Why it's wrong here
Default VPC uses Auto mode subnets with non-overlapping ranges; overlap is unlikely but would cause routing issues, not firewall.
✗
The instance has IP forwarding disabled.
Why it's wrong here
IP forwarding is required only for instances acting as routers, not for general communication.
✗
The default VPC does not have a default route to the internet.
Why it's wrong here
Default VPC has a default route (0.0.0.0/0) to the internet via the default internet gateway.
✓
The firewall rules in the VPC are blocking ingress traffic from the on-premises network.
Why this is correct
Default firewall rules allow only certain ingress; ICMP from on-premises is not allowed by default.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a configured VPN tunnel automatically allows all traffic, when in reality firewall rules (ingress) and routes must be explicitly configured to permit communication between VPC and on-premises networks.
Detailed technical explanation
How to think about this question
Cloud VPN creates a tunnel using IPsec (IKEv1 or IKEv2) and requires matching firewall rules for both ingress and egress traffic. Even with a valid tunnel and correct routes, Google Cloud VPC firewall rules are stateful for outbound connections but require explicit ingress rules for inbound traffic; ICMP echo requests (ping) from on-premises are ingress traffic that must be allowed by a firewall rule with a higher priority than the implicit deny-all ingress rule. In practice, many engineers forget that VPN traffic is subject to the same firewall rules as any other traffic, leading to this common misconfiguration.
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 a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The firewall rules in the VPC are blocking ingress traffic from the on-premises network. — Option D is correct because, by default, the default VPC includes firewall rules that allow outbound traffic but restrict inbound traffic. Even though the Cloud VPN tunnel is established, the VPC firewall rules block ingress traffic from the on-premises network (e.g., ICMP for ping). To allow communication, a firewall rule must explicitly permit ingress traffic from the on-premises IP range or the remote tunnel IP.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.