The answer is to use a policy-based route to redirect traffic from subnet A to subnet B through the firewall’s IP in subnet C. This is correct because policy-based routes allow you to define a forwarding rule that matches traffic based on specific source and destination IP ranges, then forces that traffic to a next-hop instance—in this case, the firewall appliance. Since the firewall has source/destination check disabled, it can receive the traffic, inspect it, and forward it to the final destination without being dropped. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how policy-based routes differ from static routes, which cannot selectively redirect inter-subnet traffic; a common trap is assuming a simple static route will work, but that would only affect traffic leaving the VPC. Memory tip: think of policy-based routes as “traffic cops” that force specific pairs of subnets to take a detour through inspection, while static routes are just highway signs for general direction.
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. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 several subnets and wants to force traffic between two specific subnets (A and B) to be inspected by a third-party firewall appliance in a separate subnet (C). The firewall has source/destination check disabled. What is the best way to route traffic from A to B through C?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a policy-based route to redirect traffic from A to B to the firewall's IP.
Policy-based routes allow you to define a forwarding rule that matches traffic based on source and destination IP ranges, then sends it to a next-hop instance (the firewall). Since the firewall has source/destination check disabled, it can forward the inspected traffic to the final destination. This is the only option that directly forces traffic between subnets A and B through the firewall in subnet C.
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.
✗
Use Cloud NAT to route traffic through the firewall.
Why it's wrong here
Cloud NAT is for outbound internet access, not for traffic steering within VPC.
✗
Use a custom route with a lower priority for the destination subnet.
Why it's wrong here
Custom routes do not allow source-based steering.
✗
Use VPC flow logs to monitor traffic.
Why it's wrong here
Flow logs only monitor, they do not steer traffic.
✓
Use a policy-based route to redirect traffic from A to B to the firewall's IP.
Why this is correct
Policy-based routes can match source and destination and redirect to a next hop.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between policy-based routes (which match on source and destination) and static routes (which match only on destination), leading candidates to incorrectly choose a custom static route (Option B) when a policy-based route is required.
Detailed technical explanation
How to think about this question
Policy-based routes in GCP are implemented as forwarding rules that match on source and destination IP ranges (and optionally protocols/ports) and redirect traffic to a VM instance (next-hop) that must have IP forwarding enabled and source/destination check disabled. This is distinct from static routes, which only match on destination. In practice, this allows asymmetric routing scenarios where the firewall inspects traffic in one direction while return traffic can follow a different path, as long as the firewall is configured to handle it.
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.
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: Use a policy-based route to redirect traffic from A to B to the firewall's IP. — Policy-based routes allow you to define a forwarding rule that matches traffic based on source and destination IP ranges, then sends it to a next-hop instance (the firewall). Since the firewall has source/destination check disabled, it can forward the inspected traffic to the final destination. This is the only option that directly forces traffic between subnets A and B through the firewall in subnet C.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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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