The correct answer involves checking the RDS security group for the database port, analyzing VPC Flow Logs, and verifying network ACL rules. This is because when troubleshooting EC2 to RDS connectivity issues, security groups act as a virtual firewall at the instance level, and even if the EC2 security group is allowed, the RDS security group must explicitly permit inbound traffic on the specific database port—such as 3306 for MySQL—otherwise the connection is silently dropped. VPC Flow Logs capture metadata about IP traffic, allowing you to confirm whether packets are being accepted or rejected by security groups or NACLs, while network ACLs, though defaulted to allow all traffic, must still permit both inbound and outbound traffic on ephemeral ports for the response to reach the EC2 instance. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to isolate layer 4 connectivity failures in a multi-tier architecture, often using a trap where candidates overlook the explicit port requirement in the security group rule. Remember the memory tip: "Port, Log, and ACL—three layers to unravel the riddle."
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 experiencing intermittent connectivity issues between an EC2 instance and an RDS database. The EC2 instance is in a public subnet, and the RDS instance is in a private subnet. The security group for the RDS instance allows inbound traffic from the EC2 instance's security group. The network ACLs are default (all traffic allowed). Which THREE steps should the engineer take to troubleshoot the connectivity issue? (Choose THREE.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable VPC Flow Logs for the subnets involved and analyze logs for dropped packets.
Option A is correct because VPC Flow Logs capture traffic. Option C is correct because the database port must be open. Option D is correct because NACL rules must allow both inbound and outbound. Option B is wrong because the issue is not DNS. Option E is wrong because the security group already allows the EC2 security group.
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.
✓
Enable VPC Flow Logs for the subnets involved and analyze logs for dropped packets.
Check the RDS security group for any deny rules that might block the EC2 instance.
Why it's wrong here
Security groups only have allow rules.
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 DOP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Incident and Event Response — This question tests Incident and Event Response — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Enable VPC Flow Logs for the subnets involved and analyze logs for dropped packets. — Option A is correct because VPC Flow Logs capture traffic. Option C is correct because the database port must be open. Option D is correct because NACL rules must allow both inbound and outbound. Option B is wrong because the issue is not DNS. Option E is wrong because the security group already allows the EC2 security group.
What should I do if I get this DOP-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 DOP-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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Question Discussion
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