PCNE Implementing network security Practice Question
This PCNE practice question tests your understanding of implementing network security. 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 network engineer is troubleshooting connectivity between two VPCs that are peered. The VPC flow logs show traffic being dropped. Firewall rules are correctly configured. Which TWO actions should the engineer take to identify the cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Packet Mirroring to capture traffic on both sides and compare.
Packet Mirroring allows you to capture and inspect actual traffic at the packet level on both sides of the VPC peering connection. Since firewall rules are correctly configured but flow logs still show drops, the issue is likely at a lower layer (e.g., routing, MTU, or asymmetric traffic). By comparing mirrored packets, you can see if traffic is actually reaching the destination interface and whether responses are being sent back, which flow logs alone cannot reveal.
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.
✗
Ensure that the subnets in both VPCs don't overlap.
Why it's wrong here
Overlapping subnets are not allowed in peering, but that would prevent peering, not drops.
✓
Use Packet Mirroring to capture traffic on both sides and compare.
Why this is correct
Packet Mirroring can help identify if traffic is reaching the destination instance.
Related concept
Read the scenario before looking for a memorised answer.
✓
Check for asymmetric routing by reviewing the VPC peering routes and Cloud Router sessions.
Why this is correct
Asymmetric routing can cause drops.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable the firewall rules to see if traffic flows.
Why it's wrong here
This would bypass security and not pinpoint the issue.
✗
Verify that the VPCs are in the same project.
Why it's wrong here
VPC peering can work across projects.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that flow logs provide enough detail to diagnose all connectivity issues, when in fact they only show summary statistics and cannot capture packet-level details needed to identify asymmetric routing or MTU problems.
Detailed technical explanation
How to think about this question
Packet Mirroring works by duplicating packets from a source VM or network interface and forwarding them to a collector for analysis, enabling you to see the full packet including headers and payload. In VPC peering scenarios, asymmetric routing can occur when return traffic takes a different path than forward traffic, causing stateful firewalls to drop packets; comparing mirrored traffic from both sides helps detect this asymmetry. Flow logs only show metadata (5-tuple, bytes, action) and cannot reveal packet fragmentation, MTU issues, or whether the traffic actually reached the destination NIC.
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.
Implementing network security — This question tests Implementing network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Packet Mirroring to capture traffic on both sides and compare. — Packet Mirroring allows you to capture and inspect actual traffic at the packet level on both sides of the VPC peering connection. Since firewall rules are correctly configured but flow logs still show drops, the issue is likely at a lower layer (e.g., routing, MTU, or asymmetric traffic). By comparing mirrored packets, you can see if traffic is actually reaching the destination interface and whether responses are being sent back, which flow logs alone cannot reveal.
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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