- A
The on-premises router does not have a route for the GCP subnet (10.0.1.0/24) pointing to the VPN tunnel.
Without a return route, the on-premises server sends replies through the default route (likely internet), causing asymmetric routing and packet drop.
- B
The on-premises server is not configured with a default gateway pointing to the on-premises gateway.
Why wrong: If the server had no default gateway, it wouldn't be able to reach the on-premises gateway either.
- C
The Cloud VPN tunnel is not configured with an IKE version supported by the on-premises device.
Why wrong: The tunnel is established (VM can ping on-premises gateway), so IKE configuration is fine.
- D
A firewall rule on the GCP VPC is blocking ICMP traffic from 192.168.1.100.
Why wrong: The firewall rule allows ingress from the entire on-premises subnet, so this is not the cause.
Quick Answer
The answer is that the on-premises router lacks a return route for the GCP subnet 10.0.1.0/24 pointing to the VPN tunnel. This is the most likely cause because a one-way ping from the GCP VM to the on-premises server indicates the forward path works, but the return traffic from the server to the VM is silently dropped by the on-premises router, which has no knowledge of how to reach 10.0.1.0/24. For the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of asymmetric routing in hybrid cloud VPN troubleshooting—a common trap is assuming BGP session establishment guarantees bidirectional reachability. Remember that BGP only exchanges routes that are explicitly advertised; if the on-premises router does not advertise or install a route for the GCP subnet, packets from the server will have no path back. A useful memory tip: "Ping the gateway, not the host—check the return route first."
PCNE Configuring network services Practice Question
This PCNE practice question tests your understanding of configuring network services. 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.
Your company has deployed a hybrid cloud environment with a Cloud VPN tunnel between Google Cloud VPC and an on-premises data center. The VPC has a custom mode with subnet 10.0.1.0/24 in us-east1. On-premises uses subnet 192.168.1.0/24. The VPN tunnel is established using dynamic routing (BGP). Both sides advertise the correct prefixes. A Compute Engine VM in the VPC (10.0.1.10) can ping the on-premises gateway (192.168.1.1), but cannot ping a server on-premises (192.168.1.100). The on-premises network team confirms that 192.168.1.100 is reachable from the on-premises gateway. Firewall rules in GCP allow ingress from 192.168.1.0/24 to all VMs. 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 choices
Why each option matters
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 router does not have a route for the GCP subnet (10.0.1.0/24) pointing to the VPN tunnel.
The correct answer is A. Since the VM can ping the on-premises gateway (192.168.1.1) but not the server (192.168.1.100), the VPN tunnel and BGP session are working, and GCP has the correct route. The issue is that the on-premises router is not advertising or does not have a route for the GCP subnet 10.0.1.0/24 pointing back to the VPN tunnel, so return traffic from the server to the VM is dropped. Without this route, the on-premises router cannot forward packets destined for 10.0.1.10 back through the VPN.
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 on-premises router does not have a route for the GCP subnet (10.0.1.0/24) pointing to the VPN tunnel.
Why this is correct
Without a return route, the on-premises server sends replies through the default route (likely internet), causing asymmetric routing and packet drop.
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 on-premises server is not configured with a default gateway pointing to the on-premises gateway.
Why it's wrong here
If the server had no default gateway, it wouldn't be able to reach the on-premises gateway either.
- ✗
The Cloud VPN tunnel is not configured with an IKE version supported by the on-premises device.
- ✗
A firewall rule on the GCP VPC is blocking ICMP traffic from 192.168.1.100.
Why it's wrong here
The firewall rule allows ingress from the entire on-premises subnet, so this is not the cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a successful ping to the remote gateway proves full bidirectional connectivity, but the trap here is that the gateway responds from its own IP stack, not from behind it, so a missing return route for the GCP subnet on the on-premises router breaks traffic to hosts beyond the gateway.
Detailed technical explanation
How to think about this question
In a BGP-based dynamic VPN, each side must advertise its prefixes to the peer. The on-premises router likely advertises 192.168.1.0/24 to GCP, which is why GCP knows how to reach the server. However, if the on-premises router does not have a route for 10.0.1.0/24 (either via BGP learned from GCP or a static route), it will drop return packets from 192.168.1.100 destined to 10.0.1.10. This is a classic asymmetric routing issue where the forward path works but the reverse path fails. The ping to the gateway succeeds because the gateway's own IP is local to the on-premises router, so it responds directly without needing a route back to GCP.
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.
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FAQ
Questions learners often ask
What does this PCNE question test?
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: The on-premises router does not have a route for the GCP subnet (10.0.1.0/24) pointing to the VPN tunnel. — The correct answer is A. Since the VM can ping the on-premises gateway (192.168.1.1) but not the server (192.168.1.100), the VPN tunnel and BGP session are working, and GCP has the correct route. The issue is that the on-premises router is not advertising or does not have a route for the GCP subnet 10.0.1.0/24 pointing back to the VPN tunnel, so return traffic from the server to the VM is dropped. Without this route, the on-premises router cannot forward packets destined for 10.0.1.10 back through the VPN.
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
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Last reviewed: Jun 30, 2026
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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