- A
The RDS instance does not have public DNS resolution enabled.
Why wrong: Not required for VPC peering connectivity.
- B
The network ACL for the private subnet is blocking inbound traffic.
Why wrong: Network ACLs are stateless; they must allow both inbound and outbound traffic, but the issue is likely routing.
- C
The route tables in each VPC do not have routes to the peered VPC CIDR.
Without proper routes, traffic cannot flow across the VPC peering connection.
- D
The security group outbound rules on the EC2 instance are blocking traffic.
Why wrong: Security groups are stateful; outbound traffic is automatically allowed if inbound is allowed.
Question 67 of 1,546
Troubleshoot VPC Peering Connectivity — Route Table Mistakes | AWS SysOps Associate Explained
This SOA-C02 practice question tests your understanding of reliability and business continuity. 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 runs a web application on EC2 instances in a private subnet. The application needs to connect to an RDS database in a different VPC. The VPCs are peered. The SysOps Administrator is troubleshooting connectivity issues. The RDS security group allows inbound traffic from the EC2 security group, but connections still fail. What could be the issue?
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 route tables in each VPC do not have routes to the peered VPC CIDR.
Option C is correct because for traffic to flow between peered VPCs, each VPC's route table must have a route pointing to the CIDR block of the other VPC. Without these routes, packets from the EC2 instance in VPC A destined for the RDS database in VPC B will be dropped, even if security groups and network ACLs are permissive. The SysOps Administrator must add a route in the private subnet's route table for the RDS VPC's CIDR, and a corresponding route in the RDS VPC's route table for the EC2 VPC's CIDR, both pointing to the VPC peering connection.
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 RDS instance does not have public DNS resolution enabled.
Why it's wrong here
Not required for VPC peering connectivity.
- ✗
The network ACL for the private subnet is blocking inbound traffic.
Why it's wrong here
Network ACLs are stateless; they must allow both inbound and outbound traffic, but the issue is likely routing.
- ✓
The route tables in each VPC do not have routes to the peered VPC CIDR.
Why this is correct
Without proper routes, traffic cannot flow across the VPC peering connection.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The security group outbound rules on the EC2 instance are blocking traffic.
Why it's wrong here
Security groups are stateful; outbound traffic is automatically allowed if inbound is allowed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume security groups or network ACLs are the sole cause of connectivity failures in peered VPCs, overlooking the mandatory route table entries required for traffic to traverse the peering connection.
Detailed technical explanation
How to think about this question
VPC peering does not automatically propagate routes; you must manually add static routes in both VPC route tables pointing to the peering connection. Additionally, security groups across peered VPCs can reference each other only if the VPCs are in the same AWS Region and the security group IDs are in the same account or have been shared; otherwise, you must use CIDR-based rules. A common real-world scenario is when the EC2 instance's subnet route table lacks the peered VPC route, causing packets to be sent to the internet gateway or a NAT device instead of the peering connection, 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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this SOA-C02 question test?
Reliability and Business Continuity — This question tests Reliability and Business Continuity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The route tables in each VPC do not have routes to the peered VPC CIDR. — Option C is correct because for traffic to flow between peered VPCs, each VPC's route table must have a route pointing to the CIDR block of the other VPC. Without these routes, packets from the EC2 instance in VPC A destined for the RDS database in VPC B will be dropped, even if security groups and network ACLs are permissive. The SysOps Administrator must add a route in the private subnet's route table for the RDS VPC's CIDR, and a corresponding route in the RDS VPC's route table for the EC2 VPC's CIDR, both pointing to the VPC peering connection.
What should I do if I get this SOA-C02 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.
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Last reviewed: Jul 4, 2026
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