- A
The spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering.
The spoke VPC's subnet routes are not advertised to the hub via VPC peering. In Google Cloud, VPC peering does not automatically exchange custom subnet routes unless export/import settings are configured. The spoke's subnet routes must be exported from the spoke and imported by the hub. Without that, the hub has no route back to the spoke's source IP, causing asymmetric routing when the firewall in the hub tries to return traffic to the spoke. Thus packets are dropped.
- B
The spoke VPC does not have a route to the hub's internal load balancer IP via the peering connection.
Why wrong: The spoke VPC does need a route to the hub's internal TCP load balancer IP via the peering connection. However, with proper VPC peering configuration, the spoke should have this route. The issue is not about the spoke's route to the hub's IP, but rather the hub's route back to the spoke.
- C
The internal TCP load balancer's forwarding rule is misconfigured, pointing to the wrong target.
Why wrong: If the forwarding rule were misconfigured, it would likely affect traffic from all spokes equally, not just one specific spoke. The problem is isolated to one spoke, so it is more probable that the root cause is a routing issue.
- D
The hub VPC uses Cloud NAT, which is not compatible with VPC peering.
Why wrong: Cloud NAT is used for outbound internet access from instances without external IPs. It is not directly related to traffic flow between peered VPCs and does not cause dropped traffic in this scenario.
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 company is designing a hub-and-spoke VPC architecture in Google Cloud. The hub VPC hosts a set of shared services, including a third-party firewall appliance (NGFW) in a managed instance group behind a TCP load balancer. Spoke VPCs need to send traffic to the hub's internal TCP load balancer IP (10.0.0.10) for inspection. The firewall appliance inspects traffic and forwards it to the final destination. The network team notices that traffic from one spoke to the load balancer is being dropped. They have verified that VPC peering is established, routes are propagated, and firewall rules allow the traffic. What is the most likely cause of the dropped traffic?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering.
The most likely cause is that the spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering. Although firewall rules and initial routing allow traffic from the spoke to reach the hub's internal TCP load balancer, the hub's firewall appliance needs a route back to the spoke's source IP range to forward return traffic. Without the spoke advertising its subnet routes, the hub cannot return packets, resulting in asymmetric routing and dropped 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.
- ✓
The spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering.
Why this is correct
The spoke VPC's subnet routes are not advertised to the hub via VPC peering. In Google Cloud, VPC peering does not automatically exchange custom subnet routes unless export/import settings are configured. The spoke's subnet routes must be exported from the spoke and imported by the hub. Without that, the hub has no route back to the spoke's source IP, causing asymmetric routing when the firewall in the hub tries to return traffic to the spoke. Thus packets are dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The spoke VPC does not have a route to the hub's internal load balancer IP via the peering connection.
Why it's wrong here
The spoke VPC does need a route to the hub's internal TCP load balancer IP via the peering connection. However, with proper VPC peering configuration, the spoke should have this route. The issue is not about the spoke's route to the hub's IP, but rather the hub's route back to the spoke.
- ✗
The internal TCP load balancer's forwarding rule is misconfigured, pointing to the wrong target.
Why it's wrong here
If the forwarding rule were misconfigured, it would likely affect traffic from all spokes equally, not just one specific spoke. The problem is isolated to one spoke, so it is more probable that the root cause is a routing issue.
- ✗
The hub VPC uses Cloud NAT, which is not compatible with VPC peering.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates assume VPC peering automatically advertises all subnet routes bidirectionally. However, if custom export filters are applied or the spoke's subnet routes are not exported, the hub lacks return path routes, causing traffic drops even though outbound traffic appears to work.
Trap categories for this question
Scenario analysis trap
Cloud NAT is used for outbound internet access from instances without external IPs. It is not directly related to traffic flow between peered VPCs and does not cause dropped traffic in this scenario.
Detailed technical explanation
How to think about this question
VPC peering does not automatically exchange custom subnet routes; only the subnet ranges of each VPC are exchanged by default. If a spoke VPC has additional custom routes (e.g., for on-premises connectivity or other subnets), those must be explicitly advertised using custom route exchange. In a hub-and-spoke design with a firewall appliance, the hub must know the spoke's source IP ranges to route return traffic back through the appliance; otherwise, the appliance's response packets are dropped due to no return route.
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.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
What to study next
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FAQ
Questions learners often ask
What does this PCNE question test?
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: The spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering. — The most likely cause is that the spoke VPC's subnet routes are not advertised to the hub VPC via VPC peering. Although firewall rules and initial routing allow traffic from the spoke to reach the hub's internal TCP load balancer, the hub's firewall appliance needs a route back to the spoke's source IP range to forward return traffic. Without the spoke advertising its subnet routes, the hub cannot return packets, resulting in asymmetric routing and dropped 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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This PCNE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCNE exam.
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