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 is troubleshooting connectivity between an on-premises network and a GCP VPC over a Cloud VPN tunnel with dynamic routing (BGP). The tunnel is established and BGP session is up, but on-premises hosts cannot reach instances in the VPC. What should the engineer check first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The firewall rules in the VPC allowing incoming traffic from the on-premises CIDR.
Option C is correct because even with BGP routes, the VPC firewall rules must permit incoming traffic from the on-premises CIDR. If no appropriate ingress rule exists, traffic will be blocked. Option A is incorrect because MTU might cause packet loss but not complete failure. Option B is incorrect because advertised routes from the on-premises router are being learned (BGP is up). Option D is incorrect because the VPNgateway's external IP is necessary for the tunnel but not the immediate cause of connectivity failure.
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 advertised route from the on-premises router is a default route.
Why it's wrong here
BGP is established, so routes are exchanged; whether it is default or not, on-premises can still initiate traffic.
✗
The MTU size of the VPN tunnel.
Why it's wrong here
MTU mismatch can cause issues but is less likely than firewall rules.
✗
The Cloud VPN gateway is assigned an external IP address.
Why it's wrong here
The gateway's external IP is required for the tunnel; if the tunnel is up, it must have one.
✓
The firewall rules in the VPC allowing incoming traffic from the on-premises CIDR.
Why this is correct
Firewall rules control inbound traffic; without an allow rule, traffic is denied.
Clue confirmation
The clue word "first" in the question point toward this answer.
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 firewall rules in the VPC allowing incoming traffic from the on-premises CIDR. — Option C is correct because even with BGP routes, the VPC firewall rules must permit incoming traffic from the on-premises CIDR. If no appropriate ingress rule exists, traffic will be blocked. Option A is incorrect because MTU might cause packet loss but not complete failure. Option B is incorrect because advertised routes from the on-premises router are being learned (BGP is up). Option D is incorrect because the VPN gateway's external IP is necessary for the tunnel but not the immediate cause of connectivity failure.
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.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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