The answer is the on-premises firewall’s idle timeout killing the TCP session. When the Compute Engine instances reboot, they establish fresh connections to the database, but once those connections sit idle—no keepalives or application traffic—the firewall’s stateful inspection drops the session, causing intermittent VPN connectivity drops. This is a classic trap on the Google Professional Cloud Network Engineer exam: the Cloud VPN tunnel and BGP session remain established, so candidates often blame routing or the tunnel itself, but the real culprit is a layer-4 firewall timeout on the on-premises side. The exam tests your ability to distinguish between network-layer stability (VPN/BGP) and application-layer session state. Remember the mnemonic “Tunnel up, traffic down—check the firewall’s frown,” meaning if the VPN is green but traffic dies after idle, look for a timeout on the on-premises firewall.
PCNE Practice Question: Designing, planning, and prototyping a GCP network
This PCNE practice question tests your understanding of designing, planning, and prototyping a gcp network. 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 company has a VPC with a subnet 10.0.1.0/24 in us-central1. They have deployed Compute Engine instances that need to communicate with an on-premises database via a Cloud VPN tunnel using BGP. The on-premises network advertises the database subnet 192.168.0.0/16. The instances can reach the database for a few minutes after reboot, but then connectivity drops. The Cloud VPN logs show no errors. The BGP session remains established. What is the most likely issue?
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 firewall has an idle timeout that kills the TCP session.
The on-premises firewall is likely configured with an idle timeout that terminates TCP sessions when no traffic is exchanged for a certain period. After the instances reboot, they initiate new connections that work briefly, but once the session becomes idle (e.g., no keepalives or application traffic), the firewall drops the stateful session, causing connectivity loss. The Cloud VPN and BGP session remain up, indicating the issue is at the application or firewall layer, not the tunnel or routing.
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 firewall has an idle timeout that kills the TCP session.
Why this is correct
After a period of inactivity, the firewall drops the session; reboot resets it.
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 GCP route to the on-premises database is being preempted by a more specific route.
Why it's wrong here
That would cause persistent failure, not intermittent.
✗
The VPN tunnel's IKE session expires.
Why it's wrong here
If the IKE session expired, the tunnel would drop, but logs show no errors.
✗
The BGP session is flapping.
Why it's wrong here
Logs show no errors and session remains established.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on routing or VPN tunnel issues (B, C, D) because the problem involves BGP and Cloud VPN, but the key clue is that connectivity drops after a few minutes while the tunnel and BGP remain healthy, pointing to a session timeout at the firewall layer rather than a network-layer failure.
Trap categories for this question
Command / output trap
If the IKE session expired, the tunnel would drop, but logs show no errors.
Detailed technical explanation
How to think about this question
Stateful firewalls track TCP sessions using a 5-tuple (source IP, destination IP, source port, destination port, protocol) and enforce idle timeouts typically ranging from 60 to 600 seconds (RFC 5382 recommends 2 hours for TCP, but many enterprise firewalls use shorter values). Without application-layer keepalives or TCP keepalive probes (default 2 hours on Linux), the firewall may remove the session entry, causing subsequent packets to be dropped as they appear to be part of a non-existent connection. This is a common issue in hybrid cloud scenarios where on-premises firewalls are not configured to accommodate long-lived idle connections.
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.
Designing, planning, and prototyping a GCP network — This question tests Designing, planning, and prototyping a GCP network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The on-premises firewall has an idle timeout that kills the TCP session. — The on-premises firewall is likely configured with an idle timeout that terminates TCP sessions when no traffic is exchanged for a certain period. After the instances reboot, they initiate new connections that work briefly, but once the session becomes idle (e.g., no keepalives or application traffic), the firewall drops the stateful session, causing connectivity loss. The Cloud VPN and BGP session remain up, indicating the issue is at the application or firewall layer, not the tunnel or routing.
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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