- A
Configure an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router.
This prevents the on-premises router from injecting a default route, eliminating the asymmetric routing issue.
- B
Enable global routing on the VPC and create a more specific static route for 172.16.1.2/32 with next-hop as the VPN tunnel.
Why wrong: Global routing is for inter-region communication, not for this problem. Adding a static route would not override the BGP-learned default route on the on-premises side.
- C
Create a second Cloud VPN tunnel from a different region and establish a new BGP session to load balance traffic.
Why wrong: This does not address the route preference issue; it adds complexity and possibly more routing problems.
- D
Change the Cloud Router's BGP advertise-mode to 'custom' and advertise only the subnets that contain migrated VMs.
Why wrong: This controls what Google Cloud advertises to on-premises, not what it receives. The issue is the on-premises router sending a default route.
Quick Answer
The answer is to configure an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router. This resolves the asymmetric routing issue because when the on-premises router learns a default route via the Cloud VPN, it sends traffic for 172.16.1.2 over the tunnel, but the VM’s return traffic follows the VPC’s default route to the internet gateway instead of back through the VPN, causing packet drops. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how BGP route advertisements interact with VPC default routes in hybrid networking, and it’s a common trap where candidates mistakenly try to modify the VPC’s default route or add custom advertisements. The key insight is that the on-premises router should not receive a default route from the cloud, forcing it to use the more specific /32 BGP route for return traffic. Memory tip: “Filter the default, keep the specific” — always block unwanted default routes from BGP peers to prevent asymmetric paths.
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. 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 is migrating its on-premises data center to Google Cloud. They currently have a Cloud VPN tunnel with dynamic routing (BGP) connecting their on-premises router (ASN 65001) to a Cloud Router in us-central1 (ASN 64512). The on-premises network uses IP range 10.0.0.0/8, and the Google Cloud VPC uses 172.16.0.0/12. After migration, they notice intermittent connectivity issues: traffic from on-premises to a new VM (172.16.1.2) is sometimes dropped, while other VMs in the same subnet work fine. The VM 172.16.1.2 is fine when accessed from other Google Cloud VMs. The team suspects asymmetric routing. Investigation shows that the on-premises router receives two routes for 172.16.1.2/32: one with next-hop as the Cloud VPN tunnel and another with next-hop as the internet (default route). No custom route advertisements are configured on the Cloud Router. The VPC has a default route (0.0.0.0/0) pointing to the internet gateway. What should the network engineer do to resolve the issue without breaking other connectivity?
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
Configure an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router.
The intermittent connectivity to 172.16.1.2 is caused by asymmetric routing: on-premises traffic uses the BGP-learned /32 route (via VPN) to reach the VM, but return traffic from the VM follows the VPC's default route (0.0.0.0/0) to the internet gateway, which drops the packet because the source IP is from the on-premises range. By configuring an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router, the on-premises router will no longer have a default route pointing to the VPN tunnel, forcing it to use the more specific /32 route for 172.16.1.2 and eliminating the asymmetric path.
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.
- ✓
Configure an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router.
Why this is correct
This prevents the on-premises router from injecting a default route, eliminating the asymmetric routing issue.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Enable global routing on the VPC and create a more specific static route for 172.16.1.2/32 with next-hop as the VPN tunnel.
Why it's wrong here
Global routing is for inter-region communication, not for this problem. Adding a static route would not override the BGP-learned default route on the on-premises side.
- ✗
Create a second Cloud VPN tunnel from a different region and establish a new BGP session to load balance traffic.
Why it's wrong here
This does not address the route preference issue; it adds complexity and possibly more routing problems.
- ✗
Change the Cloud Router's BGP advertise-mode to 'custom' and advertise only the subnets that contain migrated VMs.
Why it's wrong here
This controls what Google Cloud advertises to on-premises, not what it receives. The issue is the on-premises router sending a default route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates focus on the on-premises router's routing table (the /32 route) and assume the issue is on-premises, but the real problem is the VPC's default route causing asymmetric return traffic, which is resolved by filtering the BGP advertisement of 0.0.0.0/0 from the Cloud Router to the on-premises router.
Detailed technical explanation
How to think about this question
Asymmetric routing occurs when a packet takes one path to a destination and a different path back, which can cause stateful firewalls or NAT devices to drop the packet. In this scenario, the on-premises router learns a /32 route for 172.16.1.2 via BGP from the Cloud Router, but the VPC's default route (0.0.0.0/0) points to the internet gateway, so return traffic from the VM goes out to the internet instead of back through the VPN tunnel. BGP route filtering on the Cloud Router (using inbound route filters) can reject unwanted prefixes like 0.0.0.0/0 from being advertised to the on-premises peer, ensuring that the on-premises router only uses the VPN tunnel for the specific /32 route and not for general internet-bound traffic.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
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FAQ
Questions learners often ask
What does this PCNE question test?
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: Configure an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router. — The intermittent connectivity to 172.16.1.2 is caused by asymmetric routing: on-premises traffic uses the BGP-learned /32 route (via VPN) to reach the VM, but return traffic from the VM follows the VPC's default route (0.0.0.0/0) to the internet gateway, which drops the packet because the source IP is from the on-premises range. By configuring an inbound route filter on the Cloud Router to reject the BGP route 0.0.0.0/0 from the on-premises router, the on-premises router will no longer have a default route pointing to the VPN tunnel, forcing it to use the more specific /32 route for 172.16.1.2 and eliminating the asymmetric path.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 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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