The answer is to enable IP forwarding on the firewall appliance instance. This is necessary because, by default, a virtual machine in a VPC will drop any packet that arrives on one network interface but is destined for an IP address other than its own; enabling IP forwarding at the OS level—such as setting net.ipv4.ip_forward=1 on Linux—instructs the kernel to route those packets between interfaces, allowing the appliance to act as a next-hop and forward inspected traffic back into the VPC. On the Google Professional Cloud Security Engineer exam, this concept tests your understanding of how VPC routing and instance-level networking interact, often appearing as a trap where candidates assume custom routes alone are sufficient. A common memory tip is to think of the appliance as a router: just as a router must have packet forwarding enabled, so must the instance, or it will silently drop traffic not addressed to itself.
PCSE Configuring network security Practice Question
This PCSE practice question tests your understanding of configuring network security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 deploying a firewall appliance in a VPC to inspect traffic. They create custom routes to direct traffic to the appliance. Which step is necessary to ensure the appliance can forward traffic back?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable IP forwarding on the appliance instance
Option A is correct because the firewall appliance instance must have IP forwarding enabled at the OS level (e.g., net.ipv4.ip_forward=1 on Linux) to forward packets between its network interfaces. Without this, the instance will drop any traffic not destined for its own IP address, even if VPC routes direct packets to it. This is a prerequisite for the appliance to act as a transparent or routed next-hop in the VPC routing table.
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.
✓
Enable IP forwarding on the appliance instance
Why this is correct
This allows the instance to forward packets it receives.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a load balancer
Why it's wrong here
A load balancer is not needed for inspection; the appliance itself handles forwarding.
✗
Configure the appliance as a next hop in a route
Why it's wrong here
This is already done to direct traffic to the appliance.
✗
Assign a public IP to the appliance
Why it's wrong here
Public IP is not required for forwarding; internal IP works.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the distinction between routing configuration (next-hop routes) and the OS-level requirement to actually forward packets, trapping candidates who assume that adding a route alone is sufficient for the appliance to process traffic.
Detailed technical explanation
How to think about this question
Under the hood, IP forwarding (also called routing) is controlled by the kernel parameter net.ipv4.ip_forward (Linux) or the IPEnableRouter registry key (Windows). When a VPC route points to an instance’s private IP as the next hop, the instance receives packets with a destination IP that is not its own; without IP forwarding, the kernel discards them via an ICMP Redirect or simply drops them. In real-world scenarios, forgetting to enable IP forwarding is a common cause of asymmetric routing failures, where traffic reaches the firewall but never returns, leading to timeouts.
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.
Configuring network security — This question tests Configuring network security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable IP forwarding on the appliance instance — Option A is correct because the firewall appliance instance must have IP forwarding enabled at the OS level (e.g., net.ipv4.ip_forward=1 on Linux) to forward packets between its network interfaces. Without this, the instance will drop any traffic not destined for its own IP address, even if VPC routes direct packets to it. This is a prerequisite for the appliance to act as a transparent or routed next-hop in the VPC routing table.
What should I do if I get this PCSE 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This PCSE 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 PCSE exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.