The answer is the security group for the DB instance does not allow inbound traffic from the EC2 instance's security group. This is because security groups act as virtual firewalls controlling traffic at the instance level, and by default, all inbound traffic is denied; for an EC2 instance to connect to an RDS database, the RDS security group must have an inbound rule that explicitly permits traffic on the database port (e.g., 3306 for MySQL) from the source security group attached to the EC2 instance. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of network-level access controls within a VPC, and a common trap is assuming that being in the same VPC automatically grants connectivity—it does not, as security group rules are stateful and must be explicitly configured. Another frequent distractor is misdiagnosing the endpoint or public accessibility settings, but the core issue is almost always the missing inbound rule. Memory tip: think "SG to SG" — the RDS security group must reference the EC2 security group as the source, not an IP address.
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.
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.
Option B is correct because the most common issue is security group rules not allowing inbound traffic on the database port from the EC2 instance. Option A is wrong because the endpoint is correct. Option C is wrong because the RDS instance is in the same VPC, so no VPN is needed. Option D is wrong because public accessibility is not required for same VPC connectivity.
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
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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.
What to study next
Got this wrong? Here's your next step.
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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. — Option B is correct because the most common issue is security group rules not allowing inbound traffic on the database port from the EC2 instance. Option A is wrong because the endpoint is correct. Option C is wrong because the RDS instance is in the same VPC, so no VPN is needed. Option D is wrong because public accessibility is not required for same VPC connectivity.
What should I do if I get this DBS-C01 question wrong?
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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