The answer is that the Cloud Router is configured with custom route advertisement that only includes the two specified ranges. This is because Cloud Router’s BGP advertisement behavior is controlled by its advertisement mode: by default, it advertises all VPC subnets, but when set to custom route advertisement, it advertises only the explicitly listed IP ranges. In this scenario, the on-premises network receives only 10.0.1.0/24 and 10.0.2.0/24 because those are the only custom ranges defined, so other VPC subnets are suppressed from BGP advertisements. On the Google Professional Cloud Network Engineer exam, this tests your understanding of the distinction between the default ALL_SUBNETS mode and custom advertisement, a common trap being that candidates assume all subnets are always advertised. A helpful memory tip is “custom means curated”—if you see only specific ranges in BGP routes, think custom advertisement, not a routing or session failure.
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. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
You run the command shown in the exhibit. Your on-premises network is connected to your VPC via a Cloud Router with two BGP sessions. You notice that your on-premises network receives routes for only the two custom IP ranges (10.0.1.0/24 and 10.0.2.0/24) but not for other subnets in the VPC. 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.
The BGP interface IP addresses 169.254.x.x are not reachable from the on-premises side.
Why wrong: 169.254.x.x are link-local addresses used for BGP peering; they are automatically assigned and should be reachable if the VPN tunnel is up.
B
The BGP peer ASN 65001 is not recognized by the on-premises router.
Why wrong: ASN 65001 is a valid private ASN; if it were not recognized, BGP session would not establish, but the session is up and routes are received.
C
The advertisedGroups includes ALL_SUBNETS but the router is ignoring it because of a misconfiguration.
Why wrong: ALL_SUBNETS is present but when mode is CUSTOM, the advertisedGroups is not used; only the custom ranges are used. This is by design, not a misconfiguration.
D
The Cloud Router is configured with custom route advertisement that only includes the two specified ranges.
With advertiseMode CUSTOM, the router only advertises the explicitly listed ranges, ignoring ALL_SUBNETS unless it is the only group. The advertisedGroups includes ALL_SUBNETS but since mode is CUSTOM, only custom ranges are advertised.
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 configured with custom route advertisement that only includes the two specified ranges.
Option D is correct because the Cloud Router's custom route advertisement configuration explicitly controls which routes are advertised to on-premises via BGP. If only the two custom IP ranges (10.0.1.0/24 and 10.0.2.0/24) are included in the custom advertisement, other VPC subnets will not be advertised, even if they exist. The exhibit shows that the on-premises network receives only those two ranges, which directly matches a custom advertisement setup rather than the default ALL_SUBNETS behavior.
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 BGP interface IP addresses 169.254.x.x are not reachable from the on-premises side.
Why it's wrong here
169.254.x.x are link-local addresses used for BGP peering; they are automatically assigned and should be reachable if the VPN tunnel is up.
✗
The BGP peer ASN 65001 is not recognized by the on-premises router.
Why it's wrong here
ASN 65001 is a valid private ASN; if it were not recognized, BGP session would not establish, but the session is up and routes are received.
✗
The advertisedGroups includes ALL_SUBNETS but the router is ignoring it because of a misconfiguration.
Why it's wrong here
ALL_SUBNETS is present but when mode is CUSTOM, the advertisedGroups is not used; only the custom ranges are used. This is by design, not a misconfiguration.
✓
The Cloud Router is configured with custom route advertisement that only includes the two specified ranges.
Why this is correct
With advertiseMode CUSTOM, the router only advertises the explicitly listed ranges, ignoring ALL_SUBNETS unless it is the only group. The advertisedGroups includes ALL_SUBNETS but since mode is CUSTOM, only custom ranges are advertised.
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
The trap here is that candidates often assume BGP session establishment implies all routes are automatically exchanged, but Cloud Router's custom advertisement feature allows granular control over which prefixes are advertised, and the default ALL_SUBNETS behavior is not active when custom ranges are explicitly listed.
Detailed technical explanation
How to think about this question
Cloud Router uses BGP to advertise VPC subnet routes to on-premises networks. By default, it advertises all subnets (ALL_SUBNETS mode), but you can override this with custom route advertisements that specify exact CIDR ranges. When custom advertisements are configured, the Cloud Router sends only those prefixes in BGP UPDATE messages, even if other subnets exist in the VPC. This is a common design pattern for selective route propagation in hybrid networking scenarios, such as when you want to limit on-premises access to specific subnets for security or traffic engineering.
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: The Cloud Router is configured with custom route advertisement that only includes the two specified ranges. — Option D is correct because the Cloud Router's custom route advertisement configuration explicitly controls which routes are advertised to on-premises via BGP. If only the two custom IP ranges (10.0.1.0/24 and 10.0.2.0/24) are included in the custom advertisement, other VPC subnets will not be advertised, even if they exist. The exhibit shows that the on-premises network receives only those two ranges, which directly matches a custom advertisement setup rather than the default ALL_SUBNETS behavior.
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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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a complex on-premises network with multiple BGP AS numbers. They are connecting to GCP using Cloud VPN and wish to advertise specific prefixes. They want to ensure that only selected on-prem prefixes are advertised to GCP and no other prefixes leak. What is the best approach?
hard
A.Use route advertisements from on-prem routers and rely on GCP's route import policy
✓ B.Configure Cloud Router with custom advertised route maps to filter prefixes
C.Use VPC firewall rules to restrict incoming traffic
D.Set up a separate Cloud Router for each prefix
Why B: Cloud Router with custom advertised route maps allows you to explicitly define which on-premises prefixes are advertised to GCP via Cloud VPN. This ensures only the selected prefixes are propagated, preventing route leaks. Unlike relying on GCP's import policy, this approach gives you direct control over outbound advertisements from your on-premises network.
Last reviewed: Jun 11, 2026
Question Discussion
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