The answer is that the default VPC route for subnet-to-subnet traffic has a priority of 1000, which overrides the policy-based route because the policy-based route has a lower priority (higher numerical value, typically 2000). This happens because Google Cloud VPCs automatically create an implicit route for direct communication between subnets with a priority of 1000, and when you configure a policy-based route to redirect that traffic through a firewall appliance, the route with the numerically lower priority value (1000) wins over the higher value (2000). On the Google Professional Cloud Security Engineer exam, this tests your understanding of route priority mechanics and how policy-based route override default subnet route behavior works—a common trap is assuming policy-based routes always take precedence. The key insight is that priority values are inverted: lower numbers mean higher priority. Memory tip: think "1000 wins over 2000" or remember that the default subnet route is the "king of the hill" at priority 1000, so your policy-based route must be a lower number to dethrone it.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. 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 two subnets: subnet-a (10.0.1.0/24) and subnet-b (10.0.2.0/24). They have a firewall appliance (internal IP 10.0.1.100) that inspects all traffic between subnets. They configure a policy-based route to redirect traffic from subnet-a to subnet-b to the appliance. However, traffic from subnet-a to subnet-b still goes directly. What is missing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A VPC route with priority 1000 (direct routing between subnets) is overriding the policy-based route with a lower priority.
B is correct because VPCs have an implicit default route for subnet-to-subnet traffic with a priority of 1000, which is higher than the priority of a policy-based route (typically 2000). Since the policy-based route has a lower priority (higher numerical value), the default route takes precedence, causing traffic to bypass the firewall appliance. To override this, the policy-based route must have a priority lower than 1000 (e.g., 500) to be preferred.
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.
✗
Compute Engine instances must have a custom route table to use policy-based routes.
Why it's wrong here
Policy-based routes apply to the VPC; instances automatically use them.
✓
A VPC route with priority 1000 (direct routing between subnets) is overriding the policy-based route with a lower priority.
Why this is correct
The default VPC routes have priority 1000; if the policy-based route has a higher priority number, it is overridden.
Related concept
Read the scenario before looking for a memorised answer.
✗
A firewall rule is required to allow traffic to be redirected to the appliance.
Why it's wrong here
Firewall rules control access, not routing. The redirect is handled by routes.
✗
The policy-based route must have the next hop IP set to the appliance's external IP.
Why it's wrong here
For internal traffic, the next hop must be an internal IP, not external.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that policy-based routes automatically override default VPC routes, but the trap is that the default subnet-to-subnet route has a higher priority (1000) than the policy-based route (2000), so candidates must remember to set a lower priority on the policy-based route to make it effective.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, routes have a priority value where lower numbers indicate higher precedence; the default subnet-to-subnet route has priority 1000, while policy-based routes default to priority 2000. When a policy-based route is created, it only applies if its priority is lower than any conflicting route, meaning it must have a priority less than 1000 to override the implicit direct routing. This behavior is analogous to the longest-prefix match rule in traditional routing but with explicit priority tie-breaking for equal prefixes.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: A VPC route with priority 1000 (direct routing between subnets) is overriding the policy-based route with a lower priority. — B is correct because VPCs have an implicit default route for subnet-to-subnet traffic with a priority of 1000, which is higher than the priority of a policy-based route (typically 2000). Since the policy-based route has a lower priority (higher numerical value), the default route takes precedence, causing traffic to bypass the firewall appliance. To override this, the policy-based route must have a priority lower than 1000 (e.g., 500) to be preferred.
What should I do if I get this PCSE 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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