The correct step is to verify that subnet routes are exported from VPC-B to VPC-A, because VPC Network Peering requires explicit route export for connectivity, even when the peering state shows ACTIVE. While firewall rules may permit traffic, without the proper export of subnet routes from the peered VPC, the source VPC has no path to the destination—the routes simply do not appear in its route table. On the Google Professional Cloud Network Engineer exam, this scenario tests your understanding that default route export can be disabled, often through settings like exportSubnetRoutesWithPublicIp, and that an ACTIVE peering state only confirms the control plane is up, not that routes are shared. A common trap is assuming ACTIVE peering equals full connectivity, or misdiagnosing MTU or BGP issues. Remember the memory tip: "Peering is active, but routes are missing—check the export, not the handshake."
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.
An engineer has set up VPC Network Peering between VPC-A and VPC-B. Both VPCs have non-overlapping CIDR ranges. The peering state is ACTIVE. However, instances in VPC-A cannot reach instances in VPC-B. The engineer verified that firewall rules allow the traffic. What should the engineer check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The subnet routes are exported from VPC-B to VPC-A.
Option D is correct because VPC Network Peering requires that subnet routes be exported from the peered VPC. By default, subnet routes are automatically exported, but if the peering configuration was modified (e.g., exportSubnetRoutesWithPublicIp set to false), routes may not be exported. The engineer should verify that VPC-B exports its subnet routes to VPC-A. Option A is incorrect because MTU mismatch is unlikely to cause complete lack of connectivity. Option B is incorrect because BGP is not used in VPC peering. Option C is incorrect because the route tables themselves are not the issue; the missing routes are due to export settings.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The subnet routes are exported from VPC-B to VPC-A.
Why this is correct
Subnet routes must be exported explicitly if default export settings are changed.
MTU mismatch might cause packet drops but not complete failure.
✗
The route tables for the peered network.
Why it's wrong here
The route tables are not misconfigured; the issue is export of subnet routes.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
Implementing a Virtual Private Cloud — This question tests Implementing a Virtual Private Cloud — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The subnet routes are exported from VPC-B to VPC-A. — Option D is correct because VPC Network Peering requires that subnet routes be exported from the peered VPC. By default, subnet routes are automatically exported, but if the peering configuration was modified (e.g., exportSubnetRoutesWithPublicIp set to false), routes may not be exported. The engineer should verify that VPC-B exports its subnet routes to VPC-A. Option A is incorrect because MTU mismatch is unlikely to cause complete lack of connectivity. Option B is incorrect because BGP is not used in VPC peering. Option C is incorrect because the route tables themselves are not the issue; the missing routes are due to export settings.
What should I do if I get this PCNE question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related PCNE subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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