The answer is to add the 'db' tag to the custom route. This is correct because custom route tag-based applicability to VMs works by filtering which tagged instances can use a given route; a route tagged with 'web' only applies to VMs carrying that tag, so VMs in subnet-b tagged 'db' are excluded from the route to 172.16.0.0/16. By adding the 'db' tag, the route becomes applicable to both tagged groups, allowing all VMs to reach on-premises without altering the existing 'web' traffic. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding of how route tags interact with dynamic routes from Cloud VPN and BGP—a common trap is assuming that subnet-level routes apply to all instances in that subnet, when in fact tags override that behavior. Remember the memory tip: "Tags tie traffic; a route without the right tag is a dead end."
PCNE Implementing a Virtual Private Cloud Practice Question
This PCNE practice question tests your understanding of implementing a virtual private cloud. 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 uses a VPC with two subnets: subnet-a (10.0.1.0/24) with VMs tagged 'web', and subnet-b (10.0.2.0/24) with VMs tagged 'db'. They have a Cloud VPN tunnel to an on-premises network (172.16.0.0/16). The VPN tunnel is up and BGP is exchanging routes. A custom route for 172.16.0.0/16 with next hop VPN gateway exists, but it has a tag 'web', meaning it applies only to VMs with the 'web' tag. VMs in subnet-a can reach on-premises, but VMs in subnet-b cannot. Which step should be taken to allow subnet-b VMs to reach on-premises?
Create a new route for 172.16.0.0/16 with priority 1000 and no tag.
Why wrong: This would also work, but it introduces an unnecessary extra route and might cause confusion. Adding the tag is more precise.
B
Add the 'db' tag to the custom route.
Adding the tag will make the route applicable to VMs in subnet-b, allowing them to reach on-premises.
C
Remove the tag from the existing route.
Why wrong: Removing the tag would make the route apply to all VMs, which could be a broader change than intended. Adding the tag is more targeted.
D
Add a firewall rule to allow egress traffic from subnet-b.
Why wrong: The issue is a missing route, not a firewall rule. Firewall rules are stateful and allow return traffic; the problem is that the route is not applied.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add the 'db' tag to the custom route.
The custom route for 172.16.0.0/16 is tagged with 'web', so it only applies to VMs that have the 'web' tag. Subnet-b VMs are tagged 'db', so they do not match the route and cannot reach on-premises. Adding the 'db' tag to the route makes it apply to both tagged groups, enabling connectivity for subnet-b VMs without affecting existing traffic.
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.
✗
Create a new route for 172.16.0.0/16 with priority 1000 and no tag.
Why it's wrong here
This would also work, but it introduces an unnecessary extra route and might cause confusion. Adding the tag is more precise.
✓
Add the 'db' tag to the custom route.
Why this is correct
Adding the tag will make the route applicable to VMs in subnet-b, allowing them to reach on-premises.
Related concept
Read the scenario before looking for a memorised answer.
✗
Remove the tag from the existing route.
Why it's wrong here
Removing the tag would make the route apply to all VMs, which could be a broader change than intended. Adding the tag is more targeted.
✗
Add a firewall rule to allow egress traffic from subnet-b.
Why it's wrong here
The issue is a missing route, not a firewall rule. Firewall rules are stateful and allow return traffic; the problem is that the route is not applied.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that firewall rules are the cause of connectivity issues when the real problem is route scope or tag-based route applicability, leading candidates to incorrectly choose a firewall-related option like D.
Trap categories for this question
Similar concept trap
This would also work, but it introduces an unnecessary extra route and might cause confusion. Adding the tag is more precise.
Detailed technical explanation
How to think about this question
In Google Cloud VPC, custom routes can be tagged to restrict their application to VM instances with matching network tags; the route priority (default 1000) determines which route is used when multiple routes match the same destination. BGP-advertised routes from Cloud VPN have a default priority of 1000, so a custom route with the same priority and more specific prefix (172.16.0.0/16) will take precedence over a less specific BGP route if one exists. Adding the 'db' tag to the existing route is the most efficient fix because it avoids creating additional routes and maintains the original routing policy for 'web' VMs.
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.
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add the 'db' tag to the custom route. — The custom route for 172.16.0.0/16 is tagged with 'web', so it only applies to VMs that have the 'web' tag. Subnet-b VMs are tagged 'db', so they do not match the route and cannot reach on-premises. Adding the 'db' tag to the route makes it apply to both tagged groups, enabling connectivity for subnet-b VMs without affecting existing traffic.
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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