The answer is that Cloud Router, by default, only advertises subnets that reside in its own region. This is because Cloud Router’s default advertisement mode is limited to the regional scope in which it was created, so a router in us-central1 will only propagate the 10.0.1.0/24 subnet to the on-premises BGP peer, leaving the 10.0.2.0/24 subnet in us-east1 unadvertised. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of Cloud Router’s regional subnet advertisement behavior versus global routing, often appearing as a trap where candidates assume all VPC subnets are automatically advertised. A common mistake is to focus on firewall rules or BGP session status, but the core issue is that the router’s default settings restrict advertisements to its own region. To fix this, you would either create a Cloud Router in each region or enable global routing on the existing router. Memory tip: “Router’s region is its vision”—a Cloud Router only sees and shares subnets in its own backyard.
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.
Your company has a VPC with two subnets: 10.0.1.0/24 in us-central1 and 10.0.2.0/24 in us-east1. They have a Cloud VPN tunnel to the on-premises data center using dynamic routing (BGP). The Cloud Router was created in the us-central1 region with default settings. On-premises hosts can successfully communicate with instances in the 10.0.1.0/24 subnet, but cannot reach instances in the 10.0.2.0/24 subnet. All instances have appropriate firewall rules allowing traffic from on-premises. The BGP session is established and routes from on-premises are received in Cloud Router. What is the most likely reason for the 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
✓
Cloud Router by default only advertises subnets in its own region.
Cloud Router with default settings only advertises subnets that are in the same region as the Cloud Router itself. Since the Cloud Router was created in us-central1, it only advertises the 10.0.1.0/24 subnet to the on-premises BGP peer. The 10.0.2.0/24 subnet in us-east1 is not advertised, so on-premises hosts have no route to it, even though the BGP session is up and firewall rules are correct.
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.
✓
Cloud Router by default only advertises subnets in its own region.
Why this is correct
Subnets outside the router's region are not advertised unless 'Advertise all subnets' is enabled.
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 firewall rules in us-east1 are blocking incoming traffic from on-premises.
The VPN tunnel is only configured to route traffic for us-central1.
Why it's wrong here
The tunnel is established and BGP is working; the issue is route advertisement.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume Cloud Router automatically advertises all VPC subnets, but the default regional mode restricts advertisements to the router's own region, which is a common misconfiguration in multi-region VPN designs.
Detailed technical explanation
How to think about this question
By default, Cloud Router uses a 'Global dynamic routing mode' set to 'Regional' (not 'Global'). In regional mode, the router only learns and advertises routes for subnets in its own region. To advertise subnets from multiple regions, you must either create a Cloud Router in each region or switch the Cloud Router to global dynamic routing mode, which allows it to learn and advertise routes for all VPC subnets regardless of region. This behavior is controlled by the `bgp.advertise_mode` setting.
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.
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: Cloud Router by default only advertises subnets in its own region. — Cloud Router with default settings only advertises subnets that are in the same region as the Cloud Router itself. Since the Cloud Router was created in us-central1, it only advertises the 10.0.1.0/24 subnet to the on-premises BGP peer. The 10.0.2.0/24 subnet in us-east1 is not advertised, so on-premises hosts have no route to it, even though the BGP session is up and firewall rules are correct.
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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