The answer is to add a route on the on-premises router for the us-west1 subnets with the next hop pointing to the VPN tunnel. This is correct because, while the Cloud Router in us-east1 advertises the us-west1 subnets via BGP over the tunnel, the on-premises router does not automatically know to send traffic for those subnets into the tunnel—without an explicit static or learned route, it will follow its default route, typically sending traffic over the public internet. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of asymmetric routing and the distinction between cloud-side BGP advertisements and on-premises forwarding tables; a common trap is assuming that BGP on the Cloud Router alone ensures two-way traffic. Remember the key principle: the cloud router advertises, but the on-premises router must accept and route. Memory tip: “Cloud advertises, on-premises decides—if no route, traffic rides the public tide.”
PCNE Implementing hybrid interconnectivity Practice Question
This PCNE practice question tests your understanding of implementing hybrid interconnectivity. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 VPC with subnets in us-east1 and us-west1. They have established a Cloud VPN tunnel to their on-premises network through a Cloud Router in us-east1. They want to ensure that traffic from on-premises to resources in us-west1 uses the VPN tunnel and not the public internet. What must be configured?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a route on the on-premises router for us-west1 subnets with next hop pointing to the VPN tunnel
The correct answer is C because the on-premises router must have a route for the us-west1 subnets with the VPN tunnel as the next hop. Without this, the on-premises router will use its default route (typically the public internet) to reach us-west1, bypassing the VPN tunnel. The Cloud Router in us-east1 advertises the us-west1 subnets via BGP over the VPN tunnel, but the on-premises router must be explicitly configured to forward traffic for those subnets into the tunnel.
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 a custom dynamic route on the Cloud Router for us-west1 subnets
Why it's wrong here
This advertises the subnets to on-premises, but the on-premises router still needs a route back.
✗
Create a separate VPN tunnel from on-premises to a Cloud Router in us-west1
Why it's wrong here
A single tunnel can handle multiple subnets; a separate tunnel is not required for routing.
✓
Add a route on the on-premises router for us-west1 subnets with next hop pointing to the VPN tunnel
Why this is correct
The on-premises router must have a route for the remote subnets pointing to the VPN tunnel to forward traffic through it.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure VPC firewall rules to allow traffic from on-premises to us-west1
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the Cloud Router automatically directs traffic to the correct region, but the on-premises router must have an explicit route for the remote subnets pointing to the VPN tunnel, as the Cloud Router only advertises routes and does not control the on-premises forwarding table.
Detailed technical explanation
How to think about this question
Under the hood, the Cloud Router uses BGP to exchange routes with the on-premises router. The Cloud Router can advertise VPC subnets from any region (e.g., us-west1) over a single BGP session in us-east1, as long as those subnets are in the same VPC network. The on-premises router must have a static or dynamic route (e.g., via BGP learned routes) with the VPN tunnel as the next hop for the us-west1 prefixes; otherwise, it will use its default route to the internet. In real-world scenarios, this is often overlooked when deploying multi-region VPCs with a single VPN connection, leading to asymmetric routing or traffic egressing over the public internet.
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: Add a route on the on-premises router for us-west1 subnets with next hop pointing to the VPN tunnel — The correct answer is C because the on-premises router must have a route for the us-west1 subnets with the VPN tunnel as the next hop. Without this, the on-premises router will use its default route (typically the public internet) to reach us-west1, bypassing the VPN tunnel. The Cloud Router in us-east1 advertises the us-west1 subnets via BGP over the VPN tunnel, but the on-premises router must be explicitly configured to forward traffic for those subnets into the tunnel.
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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Question Discussion
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