The answer is to configure the transit gateway route tables to propagate a blackhole route for VPC A and VPC B CIDRs to the inspection VPC attachment, then have the firewall forward traffic. This forces cross-VPC traffic through the inspection VPC because the blackhole route in the transit gateway table directs all traffic destined for the other VPC’s CIDR to the inspection attachment, where the firewall inspects and then re-injects the traffic. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of transit gateway route tables and centralized inspection architectures, often appearing as a distractor where candidates mistakenly choose VPC peering or NAT gateways. A common trap is assuming Security Groups can inspect cross-VPC traffic, but they are stateful and per-resource, not designed for centralized inspection. Remember the memory tip: “Blackhole to inspect, then forward—don’t peer or NAT to correct.”
SOA-C02 Reliability and Business Continuity Practice Question
This SOA-C02 practice question tests your understanding of reliability and business continuity. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 has a production AWS account with multiple VPCs connected via a transit gateway. The security team requires that all cross-VPC traffic be inspected by a centralized network firewall appliance. The firewall is deployed in a dedicated inspection VPC. The SysOps administrator must ensure that traffic from VPC A to VPC B is routed through the inspection VPC. Which configuration achieves this?
Create a VPC peering connection between VPC A and VPC B, and use route tables to send traffic through the inspection VPC.
Why wrong: VPC peering is direct and cannot be forced through a third VPC.
B
Configure the transit gateway route tables to propagate a blackhole route for VPC A and VPC B CIDRs to the inspection VPC attachment, then have the firewall forward traffic.
TGW route tables can force traffic through inspection VPC.
C
Use security groups in the inspection VPC to filter traffic between VPC A and VPC B.
Why wrong: Security groups are per-instance, not for routing.
D
Attach a NAT gateway in the inspection VPC and configure route tables in VPC A and B to point to the NAT gateway.
Why wrong: NAT gateway is for outbound internet, not cross-VPC.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the transit gateway route tables to propagate a blackhole route for VPC A and VPC B CIDRs to the inspection VPC attachment, then have the firewall forward traffic.
Option D is correct because a blackhole route to the inspection VPC forces traffic to go through it, and then the firewall forwards it. Option A is wrong because NAT gateways do not inspect traffic. Option B is wrong because VPC peering does not route through a central inspection point. Option C is wrong because Security Groups are stateful and not designed for centralized inspection.
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.
✗
Create a VPC peering connection between VPC A and VPC B, and use route tables to send traffic through the inspection VPC.
Why it's wrong here
VPC peering is direct and cannot be forced through a third VPC.
✓
Configure the transit gateway route tables to propagate a blackhole route for VPC A and VPC B CIDRs to the inspection VPC attachment, then have the firewall forward traffic.
Why this is correct
TGW route tables can force traffic through inspection VPC.
Use security groups in the inspection VPC to filter traffic between VPC A and VPC B.
Why it's wrong here
Security groups are per-instance, not for routing.
✗
Attach a NAT gateway in the inspection VPC and configure route tables in VPC A and B to point to the NAT gateway.
Why it's wrong here
NATgateway is for outbound internet, not cross-VPC.
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Reliability and Business Continuity — This question tests Reliability and Business Continuity — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure the transit gateway route tables to propagate a blackhole route for VPC A and VPC B CIDRs to the inspection VPC attachment, then have the firewall forward traffic. — Option D is correct because a blackhole route to the inspection VPC forces traffic to go through it, and then the firewall forwards it. Option A is wrong because NAT gateways do not inspect traffic. Option B is wrong because VPC peering does not route through a central inspection point. Option C is wrong because Security Groups are stateful and not designed for centralized inspection.
What should I do if I get this SOA-C02 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 SOA-C02 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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