- A
Delete the conflicting subnet in the VPC.
Why wrong: Deleting a production subnet is disruptive and unnecessary.
- B
Modify the on-premises BGP advertisement to use a more specific prefix (longer subnet mask) that does not overlap.
A more specific prefix will be preferred for traffic destined to that subnet, eliminating the conflict.
- C
Use route propagation with a filter in the VPC route table.
Why wrong: Route propagation is a concept within the router itself; there is no filter for propagated routes in the route table.
- D
Create a static route in the VPC with the same prefix as the overlapping route.
Why wrong: Static routes cannot override dynamic routes with the same prefix; the dynamic route remains.
- E
Use Cloud Router custom route advertisements to control which routes are learned or advertised.
Cloud Router can be configured to filter or modify BGP routes to avoid conflicts.
Quick Answer
The answer is to use Cloud Router custom route advertisements and advertise a more specific prefix from on-premises. This works because BGP route selection prioritizes the most specific (longest) prefix match, so a smaller on-premises CIDR block will override the overlapping VPC subnet for that traffic, while custom route advertisements on Cloud Router allow you to explicitly filter or modify which BGP routes are learned or advertised to prevent the conflict entirely. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of BGP route priority and Cloud Router’s control plane; a common trap is assuming a static route can override a dynamic BGP route of the same length, but the more specific prefix always wins. Remember the memory tip: “Specificity solves overlap”—the longest prefix match is your scalpel for carving out conflicting routes.
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 uses Cloud VPN with dynamic routing (BGP). The on-premises network advertises a prefix that overlaps with a subnet in the VPC. Which TWO actions can resolve this conflict? (Choose TWO.)
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Modify the on-premises BGP advertisement to use a more specific prefix (longer subnet mask) that does not overlap.
Option A: Advertise a more specific prefix from on-premises (e.g., a smaller CIDR block) to differentiate traffic. Option C: On Cloud Router, configure custom route advertisements to filter or modify the overlapping route. Option B would not work because a static route cannot override a dynamic route with the same prefix length; the more specific route wins. Option D is too drastic and unnecessary. Option E is not a native feature.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Delete the conflicting subnet in the VPC.
Why it's wrong here
Deleting a production subnet is disruptive and unnecessary.
- ✓
Modify the on-premises BGP advertisement to use a more specific prefix (longer subnet mask) that does not overlap.
Why this is correct
A more specific prefix will be preferred for traffic destined to that subnet, eliminating the conflict.
Related concept
CIDR notation defines the prefix length.
- ✗
Use route propagation with a filter in the VPC route table.
Why it's wrong here
Route propagation is a concept within the router itself; there is no filter for propagated routes in the route table.
- ✗
Create a static route in the VPC with the same prefix as the overlapping route.
Why it's wrong here
Static routes cannot override dynamic routes with the same prefix; the dynamic route remains.
- ✓
Use Cloud Router custom route advertisements to control which routes are learned or advertised.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this PCNE question test?
Configuring network services — This question tests Configuring network services — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Modify the on-premises BGP advertisement to use a more specific prefix (longer subnet mask) that does not overlap. — Option A: Advertise a more specific prefix from on-premises (e.g., a smaller CIDR block) to differentiate traffic. Option C: On Cloud Router, configure custom route advertisements to filter or modify the overlapping route. Option B would not work because a static route cannot override a dynamic route with the same prefix length; the more specific route wins. Option D is too drastic and unnecessary. Option E is not a native feature.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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Same concept, more angles
1 more ways this is tested on PCNE
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. An organization has a Dedicated Interconnect with Cloud Router configured for BGP. The on-premises network advertises a prefix that overlaps with an existing VPC subnet. How does Google Cloud handle the overlapping prefix?
hard- A.It accepts the prefix but static routes have higher priority.
- B.It accepts the prefix and gives it higher priority than the VPC subnet.
- ✓ C.It rejects the prefix and does not install a dynamic route for it.
- D.It accepts the prefix and load balances traffic between the two locations.
Why C: When a Dedicated Interconnect with Cloud Router receives a BGP advertisement for a prefix that overlaps with an existing VPC subnet, Google Cloud rejects the prefix and does not install a dynamic route for it. This is because VPC subnet routes always take precedence over dynamically learned routes, and accepting an overlapping prefix would create ambiguity in forwarding decisions. The Cloud Router enforces this behavior to maintain the integrity of the VPC's internal addressing.
Last reviewed: Jun 24, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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