DBS-C01 Deployment and Migration Practice Question
This DBS-C01 practice question tests your understanding of deployment and migration. 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.
Network Topology
After migrating a database to Amazon RDS, an application on an EC2 instance in the same VPC cannot connect. The command in the exhibit shows the endpoint. What is the most likely cause?
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.
Network Topology
A
The EC2 instance is in a different VPC and needs a VPN connection.
Why wrong: The problem states they are in the same VPC.
B
The security group for the DB instance does not allow inbound traffic from the EC2 instance's security group.
Security group rules control access; if not configured, connections are blocked.
C
The DB instance endpoint is incorrect.
Why wrong: The endpoint appears valid.
D
The DB instance is not publicly accessible.
Why wrong: Public accessibility is not required for internal VPC connectivity.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The security group for the DB instance does not allow inbound traffic from the EC2 instance's security group.
The most likely cause is that the security group for the RDS DB instance does not have an inbound rule allowing traffic from the EC2 instance's security group on the database port (typically TCP 3306 for MySQL or 5432 for PostgreSQL). Even though both resources are in the same VPC, security groups act as virtual firewalls that must explicitly permit inbound connections from the source. The command output showing the endpoint confirms the DNS resolution works, so the issue is at the network access control layer.
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 EC2 instance is in a different VPC and needs a VPN connection.
Why it's wrong here
The problem states they are in the same VPC.
✓
The security group for the DB instance does not allow inbound traffic from the EC2 instance's security group.
Why this is correct
Security group rules control access; if not configured, connections are blocked.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The DB instance endpoint is incorrect.
Why it's wrong here
The endpoint appears valid.
✗
The DB instance is not publicly accessible.
Why it's wrong here
Public accessibility is not required for internal VPC connectivity.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume that being in the same VPC guarantees connectivity, but AWS security groups are stateful and require explicit inbound rules, even for intra-VPC traffic, which is a core concept tested in the DBS-C01 exam.
Detailed technical explanation
How to think about this question
RDS security groups operate at the instance level and evaluate inbound rules based on the source IP or source security group. When using a security group as the source, AWS automatically translates the private IPs of all instances associated with that source security group, allowing dynamic scaling without manual IP updates. A common subtlety is that the security group rule must reference the EC2 instance's security group ID (not the EC2 instance's own security group name), and the rule must specify the correct database port; otherwise, the connection will be silently dropped.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
Visual reference
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Deployment and Migration — This question tests Deployment and Migration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The security group for the DB instance does not allow inbound traffic from the EC2 instance's security group. — The most likely cause is that the security group for the RDS DB instance does not have an inbound rule allowing traffic from the EC2 instance's security group on the database port (typically TCP 3306 for MySQL or 5432 for PostgreSQL). Even though both resources are in the same VPC, security groups act as virtual firewalls that must explicitly permit inbound connections from the source. The command output showing the endpoint confirms the DNS resolution works, so the issue is at the network access control layer.
What should I do if I get this DBS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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