The answer is that the new subnet range is not included in the Cloud Router's advertised routes. This is the most likely cause because Cloud VPN with dynamic routing (BGP) does not automatically propagate newly added VPC subnets to on-premises peers; the Cloud Router must be explicitly configured to advertise the new CIDR block. Even with an established BGP session and incoming routes from on-premises, the on-premises network cannot reach 10.2.0.0/16 without that subnet being listed in the Cloud Router's custom advertised routes. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that Cloud Router acts as a gatekeeper for outbound route advertisements—a common trap is assuming that adding a subnet to a VPC automatically updates BGP advertisements. Remember the memory tip: "New subnet, new advertise"—whenever you add a subnet, always check and update the Cloud Router's advertised route list to ensure connectivity.
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.
A company has a Cloud VPN tunnel with dynamic routing (BGP) connecting their on-premises network to a VPC in us-central1. They recently added a new subnet (10.2.0.0/16) to the VPC. The on-premises network still cannot reach resources in the new subnet. The Cloud Router BGP session is established and routes from on-prem are being received. 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 new subnet range is not included in the Cloud Router's advertised routes
When a new subnet is added to a VPC, the Cloud Router must be configured to advertise that subnet's CIDR range to the on-premises BGP peer. Even though the BGP session is established and routes from on-premises are received, the Cloud Router will not automatically advertise the new subnet unless its advertised route list is updated. Without the new subnet in the Cloud Router's advertised routes, the on-premises network has no path to reach 10.2.0.0/16, causing connectivity failure.
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 Cloud Router does not support more than 10 custom dynamic routes
Why it's wrong here
Cloud Router supports up to 200 custom dynamic routes per BGP session.
✓
The new subnet range is not included in the Cloud Router's advertised routes
Why this is correct
Custom dynamic routes must be added to the Cloud Router's advertised route list for the remote peer to learn them.
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.
Firewall rules would be a separate issue, not the most likely cause given routing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between route propagation (BGP session up) and route advertisement (what prefixes are actually sent), leading candidates to incorrectly assume that an established BGP session automatically advertises all VPC subnets.
Detailed technical explanation
How to think about this question
Cloud Router uses BGP to exchange routes with on-premises routers; by default, it advertises only the VPC's primary subnet ranges. When a new subnet is added, you must either add its CIDR to the Cloud Router's 'advertised IP ranges' or enable 'Advertise all subnets visible by the Cloud Router' (which includes all VPC subnets). This is analogous to using a 'network statement' under BGP in Cisco IOS to advertise a prefix. In real-world scenarios, forgetting to update BGP advertisements after subnet additions is a common cause of asymmetric routing or blackholing.
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.
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: The new subnet range is not included in the Cloud Router's advertised routes — When a new subnet is added to a VPC, the Cloud Router must be configured to advertise that subnet's CIDR range to the on-premises BGP peer. Even though the BGP session is established and routes from on-premises are received, the Cloud Router will not automatically advertise the new subnet unless its advertised route list is updated. Without the new subnet in the Cloud Router's advertised routes, the on-premises network has no path to reach 10.2.0.0/16, causing connectivity failure.
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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