The answer is a missing route table entry for the on-premises CIDR (192.168.0.0/16) pointing to the virtual private gateway. Even when the VPN tunnel status is UP and security groups allow ICMP, traffic from the VPC to the on-premises network will fail if the VPC route table lacks a specific route directing that traffic to the VGW. This is because the VPC’s default route only handles local traffic; without an explicit entry for the remote network, the VPC has no path to forward packets back to the on-premises side, breaking bidirectional communication. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of how route propagation and static routes interact with Site-to-Site VPNs—a common trap is assuming a tunnel UP status guarantees connectivity, when routing misconfigurations are often the real culprit. A useful memory tip: “Tunnel up does not mean traffic flows; check the route table for where the traffic goes.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure 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 migrating its on-premises data center to AWS. The company has a VPC with a CIDR of 10.0.0.0/16 and needs to connect its on-premises network (192.168.0.0/16) to the VPC using an AWS Site-to-Site VPN. The security engineer has configured the virtual private gateway (VGW) and the customer gateway (CGW) with the correct settings. The VPN tunnel status is UP, but the on-premises servers cannot ping the EC2 instances in the VPC. The EC2 instances have security groups that allow ICMP traffic from the on-premises network. The VPC route table has a route for the on-premises network pointing to the VGW. What is the most likely cause of the issue?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC route table does not have a route for the on-premises CIDR (192.168.0.0/16) pointing to the virtual private gateway.
Option A is correct. The VPC route table needs a route for the on-premises CIDR pointing to the VGW. If this route is missing, traffic from the VPC to on-premises will not be routed correctly. Option B is incorrect because security groups are stateful and allow return traffic. Option C is not required if the VPN is configured correctly. Option D is incorrect because the VGW is the correct target.
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 customer gateway is not configured with the correct public IP address of the VGW.
Why it's wrong here
The tunnel is UP, so the IP is correct.
✗
The VPN tunnel is not configured with the correct pre-shared key.
Why it's wrong here
The tunnel status is UP, so the key is correct.
✗
The security group for the EC2 instances does not allow inbound ICMP from the on-premises network.
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.
Trap categories for this question
Scenario analysis trap
The scenario states ICMP is allowed.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC route table does not have a route for the on-premises CIDR (192.168.0.0/16) pointing to the virtual private gateway. — Option A is correct. The VPC route table needs a route for the on-premises CIDR pointing to the VGW. If this route is missing, traffic from the VPC to on-premises will not be routed correctly. Option B is incorrect because security groups are stateful and allow return traffic. Option C is not required if the VPN is configured correctly. Option D is incorrect because the VGW is the correct target.
What should I do if I get this SCS-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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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