The answer is a subnet overlap between the on-premises range and the VPC’s auto-allocated IP range. When a Cloud Router advertises 10.0.0.0/8 from on-premises, but the VPC already contains a subnet within that same CIDR block—such as a default or custom range like 10.0.1.0/24—Google Cloud’s routing logic treats the local VPC route as more specific and preferred, causing traffic destined for the on-premises subnet to be dropped rather than forwarded. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of route priority and the fact that VPC local routes always win over dynamically learned routes, even if the on-premises range is larger. A common trap is assuming Cloud Router advertisements will override existing VPC subnets, but the key is that overlapping ranges create a conflict where the VPC silently drops traffic to avoid asymmetric routing. Memory tip: “Local wins, overlap drops”—if your on-premises CIDR touches any VPC subnet, traffic to that on-premises range will be blackholed.
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.
Refer to the exhibit. A network engineer configured a Cloud Router to advertise the on-premises subnet 10.0.0.0/8 to the VPC. However, traffic from VPC instances to 10.0.0.0/8 is being dropped. 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 subnet 10.0.0.0/8 overlaps with the VPC's auto-allocated IP range.
Option C is correct because Cloud Router uses custom route advertisements, and if the on-premises subnet 10.0.0.0/8 overlaps with the VPC's auto-allocated IP range (e.g., the default or custom subnet ranges within the VPC), Google Cloud will not install or will drop traffic for that route due to a conflict. Overlapping routes cause the VPC to prefer its own local routes, resulting in dropped traffic to the on-premises subnet.
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 advertised route has a priority that is too low.
Why it's wrong here
Priority 100 is the default and does not cause drops.
✗
The Cloud Router's ASN is private, causing routes to be rejected.
Why it's wrong here
Private ASN is acceptable and does not cause route rejection.
✓
The subnet 10.0.0.0/8 overlaps with the VPC's auto-allocated IP range.
Why this is correct
Overlap causes VPC to prefer local routes, dropping traffic destined for on-premises.
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 on-premises router is not configured to accept the advertised route.
Why it's wrong here
The route is advertised to the VPC, not the on-premises router, so that is irrelevant.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that route priority or BGP ASN issues cause traffic drops, but the trap here is that overlapping IP ranges between on-premises and VPC subnets silently cause traffic to be dropped due to VPC local route precedence, not because of BGP configuration errors.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, auto-allocated IP ranges (e.g., for default subnets or secondary ranges) are assigned from the 10.0.0.0/8 space by default, and local VPC routes always take precedence over custom advertised routes from Cloud Router. When a Cloud Router advertises a subnet that overlaps with an existing VPC subnet, the VPC's local route is preferred, and the advertised route is either not installed or is shadowed, causing traffic to be dropped if the destination is not actually in the VPC subnet. This behavior is governed by the VPC route priority system where local routes have an implicit priority of 0, while Cloud Router advertised routes have a configurable priority (default 100), making the local route always win.
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 subnet 10.0.0.0/8 overlaps with the VPC's auto-allocated IP range. — Option C is correct because Cloud Router uses custom route advertisements, and if the on-premises subnet 10.0.0.0/8 overlaps with the VPC's auto-allocated IP range (e.g., the default or custom subnet ranges within the VPC), Google Cloud will not install or will drop traffic for that route due to a conflict. Overlapping routes cause the VPC to prefer its own local routes, resulting in dropped traffic to the on-premises subnet.
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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