The answer is a missing static route on the on-premises router for 192.168.1.0/24 pointing to the VPN gateway. Even when a VPN tunnel is up but no connectivity exists, the tunnel’s establishment only confirms the control plane is working; data plane traffic still requires proper routing on both sides. Without a static route on the on-premises router directing traffic for the VPC subnet toward the VPN gateway, packets from the on-premises server at 10.0.0.50 to 192.168.1.10 are either dropped or sent to the default gateway, never entering the tunnel. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that Cloud VPN logs only show tunnel health, not routing completeness—a common trap is assuming a green tunnel guarantees reachability. Remember the memory tip: “Tunnel up, route down—packets drown.”
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. 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 organization has a site-to-site Cloud VPN connection between an on-premises network with CIDR 10.0.0.0/8 and a VPC in us-central1 with subnet 192.168.1.0/24. The VPN tunnel is established, but you cannot reach a Compute Engine instance with internal IP 192.168.1.10 from a server on-premises with IP 10.0.0.50. Cloud VPN logs show no errors. On-premises firewall rules allow all outbound traffic. What is the most likely cause of the problem?
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
✓
Missing a static route on the on-premises router for 192.168.1.0/24 pointing to the VPN gateway
The VPN tunnel is established and Cloud VPN logs show no errors, indicating the cloud side is configured correctly. However, the on-premises server at 10.0.0.50 cannot reach 192.168.1.10 because the on-premises router lacks a static route for the VPC subnet 192.168.1.0/24 pointing to the VPN gateway. Without this route, the on-premises router does not know to send traffic destined for 192.168.1.0/24 through the VPN tunnel, so packets are dropped or sent to the default gateway instead.
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.
✗
A firewall rule on the VPC blocking inbound traffic from 10.0.0.0/8
Why it's wrong here
If a firewall rule blocked traffic, the logs would typically show denied packets; the question states no errors.
✓
Missing a static route on the on-premises router for 192.168.1.0/24 pointing to the VPN gateway
Why this is correct
Without a proper route on-premises, traffic to GCP may not be forwarded to the VPN peer.
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 Compute Engine instance's OS firewall blocking ICMP
Why it's wrong here
The question does not specify the protocol; still, route issue is more likely given tunnel is up.
✗
Incorrect IAM permissions on the Cloud VPN gateway
Why it's wrong here
IAM permissions are for managing VPN resources, not for traffic flow; the tunnel is established so permissions are sufficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that a successful VPN tunnel establishment implies full bidirectional connectivity, when in fact routing must be explicitly configured on both sides for traffic to flow.
Trap categories for this question
Command / output trap
If a firewall rule blocked traffic, the logs would typically show denied packets; the question states no errors.
Detailed technical explanation
How to think about this question
In site-to-site VPNs, routing must be configured on both sides: the cloud VPC needs a static route (or dynamic via Cloud Router/BGP) for the on-premises CIDR pointing to the VPN tunnel, and the on-premises router needs a static route for the VPC subnet pointing to its VPN gateway interface. Without the on-premises route, the router performs a longest-prefix match and sends the packet out its default route (usually the internet), where it is lost. This is a common misconfiguration because the tunnel being up does not guarantee bidirectional routing—it only guarantees the IPsec security associations are established.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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 hybrid interconnectivity — This question tests Implementing hybrid interconnectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Missing a static route on the on-premises router for 192.168.1.0/24 pointing to the VPN gateway — The VPN tunnel is established and Cloud VPN logs show no errors, indicating the cloud side is configured correctly. However, the on-premises server at 10.0.0.50 cannot reach 192.168.1.10 because the on-premises router lacks a static route for the VPC subnet 192.168.1.0/24 pointing to the VPN gateway. Without this route, the on-premises router does not know to send traffic destined for 192.168.1.0/24 through the VPN tunnel, so packets are dropped or sent to the default gateway instead.
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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