SCS-C02 Infrastructure Security Practice Question
A company has a VPC with a CIDR block of 10.0.0.0/16. The company wants to connect this VPC to an on-premises network that uses the CIDR block 10.0.0.0/8. The company has set up an AWS Site-to-Site VPN connection. After configuration, the on-premises network cannot reach resources in the VPC, and the VPC cannot reach on-premises resources. What is the most likely cause of the connectivity issue?
⚠ Common exam trap
The trap here is assuming that a missing route or security group rule is the cause, when the fundamental issue is overlapping CIDR blocks that make routing impossible.
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 on-premises network CIDR block overlaps with the VPC CIDR block, causing routing conflicts.
The on-premises network CIDR 10.0.0.0/8 overlaps with the VPC CIDR 10.0.0.0/16. This overlap causes routing conflicts because the same IP range exists on both sides of the VPN. Traffic cannot be correctly routed, leading to connectivity failure. The solution is to re-address either the VPC or the on-premises network to eliminate the overlap.
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 VPC route table does not have a route for the on-premises CIDR block pointing to the virtual private gateway.
Why it's wrong here
While a missing route would cause connectivity issues, the more fundamental problem is the overlapping CIDR blocks. Even with correct routes, the overlap would cause routing ambiguity and conflicts. The question states the on-premises network uses 10.0.0.0/8, which encompasses the VPC CIDR, making routing impossible without re-addressing.
- ✗
The security group attached to the VPC resources does not allow traffic from the on-premises CIDR block.
Why it's wrong here
Security groups are stateful and would need to allow traffic, but the primary issue is the CIDR overlap. Even if security groups were correctly configured, the overlapping IP ranges would prevent proper routing. Security group misconfiguration is a possible cause but not the most likely given the explicit overlap.
- ✗
The VPN connection is using dynamic routing (BGP) instead of static routing, which is required for overlapping CIDRs.
Why it's wrong here
Dynamic routing with BGP does not resolve overlapping CIDR issues; in fact, BGP would also suffer from the same ambiguity. Overlapping CIDRs cannot be resolved by changing routing protocols. The only solution is to re-address one of the networks. This option misrepresents the capabilities of BGP.
- ✓
The on-premises network CIDR block overlaps with the VPC CIDR block, causing routing conflicts.
Why this is correct
The on-premises network uses 10.0.0.0/8, which includes the VPC CIDR 10.0.0.0/16. This overlap creates conflicting routes: traffic destined for 10.0.0.0/16 could be routed locally within the VPC or to the VPN, causing ambiguity and failure. Overlapping CIDRs are a common cause of hybrid connectivity issues and must be resolved by re-addressing one side.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
This SCS-C02 practice question is part of Courseiva's free Amazon Web Services 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 SCS-C02 exam.