Resolving Overlapping Prefixes: VPN BGP Strategies
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.)
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.
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.
By advertising a more specific prefix (longer subnet mask) from on-premises, that route will be preferred over the less specific VPC subnet route due to longest prefix match. Option E is correct: Cloud Router custom route advertisements allow you to filter or modify which routes are learned or advertised, which can be used to prevent the overlapping route from being installed. Options A, C, and D are incorrect: deleting the subnet is unnecessary; route propagation filters are not a native feature; and a static route with the same prefix length would not override a dynamic BGP route.
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.
- ✗
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.
Why this is correct
Cloud Router can be configured to filter or modify BGP routes to avoid conflicts.
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Same concept, more angles
1 more way 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.