The answer is that the Cloud Router is not advertising the correct subnet range to the on-premises router. This is the most likely cause because when Cloud VPN BGP route advertisement troubleshooting reveals a tunnel is up and BGP sessions are established, the physical and logical peering layers are healthy; intermittent connectivity to a single subnet like 10.1.0.0/16 points directly to a missing or misconfigured route advertisement. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that BGP controls route propagation independently of tunnel status—a common trap is to assume the tunnel being up guarantees all subnets are reachable. Remember that Cloud Router uses custom or learned route advertisements, and if the specific subnet range isn’t included in the advertised prefixes, the on-premises router will drop traffic for that subnet. A useful memory tip: “Tunnel up, BGP up, but subnet down? Check the advertisement list.”
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.
A company has an on-premises data center connected to GCP via Cloud VPN with dynamic routing (BGP). Recently, connectivity to a specific subnet (10.1.0.0/16) in GCP became intermittent. The VPN tunnel is up, and BGP sessions are established. 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 Cloud Router is not advertising the correct subnet range to the on-premises router.
Since the VPN tunnel is up and BGP sessions are established, the issue is not with the tunnel or BGP peering itself. Intermittent connectivity to a specific subnet (10.1.0.0/16) while other subnets remain reachable strongly indicates a route advertisement problem. The Cloud Router must be configured to advertise the correct subnet range to the on-premises router via BGP; if it is not, the on-premises router will lack a route for that subnet, causing intermittent or no connectivity.
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 shared secret is misconfigured.
Why it's wrong here
Wrong shared secret would prevent tunnel establishment.
✗
The VPN tunnel is flapping due to packet loss.
Why it's wrong here
Tunnel stability is unrelated to intermittent connectivity to a specific subnet.
✗
A firewall rule is blocking traffic from on-premises.
Why it's wrong here
Firewall rules control allowed traffic, not routing.
✓
The Cloud Router is not advertising the correct subnet range to the on-premises router.
Why this is correct
If the subnet is not advertised, on-premises traffic may not be routed correctly.
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 if the VPN tunnel and BGP session are up, all subnets must be reachable, but the trap is that route advertisement misconfiguration can cause selective subnet unreachability even when the control plane is healthy.
Detailed technical explanation
How to think about this question
In GCP, Cloud Router uses BGP to exchange routes with on-premises routers. By default, Cloud Router advertises only the VPC's primary subnet ranges unless custom route advertisements are configured. If the subnet 10.1.0.0/16 is a secondary IP range or a newly added subnet, it may not be included in the BGP advertisements unless explicitly added to the Cloud Router's advertised routes. This can lead to asymmetric routing or dropped packets when the on-premises router does not have a route for that subnet, causing intermittent connectivity depending on traffic flow and route propagation timing.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
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 Cloud Router is not advertising the correct subnet range to the on-premises router. — Since the VPN tunnel is up and BGP sessions are established, the issue is not with the tunnel or BGP peering itself. Intermittent connectivity to a specific subnet (10.1.0.0/16) while other subnets remain reachable strongly indicates a route advertisement problem. The Cloud Router must be configured to advertise the correct subnet range to the on-premises router via BGP; if it is not, the on-premises router will lack a route for that subnet, causing intermittent or no connectivity.
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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