The answer is that the on-premises network lacks a route back to the VPC subnet. This is the most likely cause because Cloud VPN relies on symmetric routing: the on-premises router must have a static or dynamic route pointing the VM’s VPC subnet back to its own VPN gateway interface. Without that return route, the on-premises server receives the packet but has no path to send the reply, causing the return traffic to be dropped silently. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of asymmetric routing pitfalls in hybrid connectivity—a common trap is assuming firewall rules or BGP status are the issue, when the real culprit is a missing on-premises route. Remember that Cloud VPN is a tunnel, not a router; both sides must advertise or statically define the remote subnet. Memory tip: “No route back, no reply—check the on-prem side for the VPC subnet entry.”
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 network engineer is troubleshooting connectivity from a VM to an on-premises server over a Cloud VPN. The VM can reach the on-premises server, but the return traffic is dropped. 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 on-premises network does not have a route back to the VPC subnet.
Option C is correct because on-premises routes must include the VPC subnet to ensure return traffic is sent to the VPNgateway. Option A is wrong because firewall rules can be separate for ingress/egress. Option B is wrong because BGP session being down would cause no connectivity. Option D is wrong because Cloud Router is needed for dynamic routing, but not having one does not cause asymmetric dropping specifically.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 on-premises network does not have a route back to the VPC subnet.
Why this is correct
Return traffic requires a route on-premises pointing to the VPNgateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
A firewall rule is blocking ingress traffic from the on-premises network.
Why it's wrong here
Firewall rules would block all traffic, not just return.
✗
The BGP session is down.
Why it's wrong here
If BGP session is down, no routes are exchanged, so no connectivity.
✗
The Cloud Router is not configured.
Why it's wrong here
Cloud Router is needed for BGP, but static routes can be used; missing Cloud Router does not cause asymmetric drop.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The on-premises network does not have a route back to the VPC subnet. — Option C is correct because on-premises routes must include the VPC subnet to ensure return traffic is sent to the VPN gateway. Option A is wrong because firewall rules can be separate for ingress/egress. Option B is wrong because BGP session being down would cause no connectivity. Option D is wrong because Cloud Router is needed for dynamic routing, but not having one does not cause asymmetric dropping specifically.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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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.
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