← DBS-C01·Amazon Web Services

Question 1,285 of 1,730

Deployment and Migration →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is the RDS security group configuration, because it only allows inbound MySQL traffic from the VPC CIDR (10.0.0.0/16) rather than from the EC2 security group itself. For an EC2 instance to connect to an RDS database via CloudFormation, the RDS security group must explicitly reference the EC2 security group as the source—not just the VPC’s IP range. This is a classic RDS security group connectivity EC2 CloudFormation trap on the AWS Certified Database Specialty DBS-C01 exam, testing your understanding that security group rules are stateful and require group-to-group references, not CIDR blocks, for cross-service access within the same VPC. Many candidates mistakenly assume that being in the same VPC or subnet is sufficient, but the exam emphasizes that security group rules must match the source of the traffic. Memory tip: think “group-to-group, not CIDR-to-group” when troubleshooting EC2 access to RDS in CloudFormation templates.

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.

Exhibit

Refer to the exhibit.

CloudFormation template snippet:

Resources:
  MyRDSInstance:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceClass: db.t3.medium
      Engine: MySQL
      MultiAZ: true
      DBSubnetGroupName: !Ref MyDBSubnetGroup
      VPCSecurityGroups:
        - !Ref MyDBSecurityGroup
      StorageType: gp2
      AllocatedStorage: 100
      BackupRetentionPeriod: 7
      DeletionProtection: true

  MyDBSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for RDS
      VpcId: !Ref MyVPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 3306
          ToPort: 3306
          CidrIp: 10.0.0.0/16

  MyDBSubnetGroup:
    Type: AWS::RDS::DBSubnetGroup
    Properties:
      DBSubnetGroupDescription: Subnet group for RDS
      SubnetIds:
        - subnet-12345678
        - subnet-87654321

  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t2.micro
      ImageId: ami-0abcdef1234567890
      SecurityGroupIds:
        - !Ref MyEC2SecurityGroup
      SubnetId: subnet-12345678

  MyEC2SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for EC2
      VpcId: !Ref MyVPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 3306
          ToPort: 3306
          CidrIp: 0.0.0.0/0
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0

Refer to the exhibit. A developer deploys this CloudFormation template. An application on the EC2 instance cannot connect to the RDS MySQL database. 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.

Question 1hardmultiple choice

Full question →

Exhibit

Refer to the exhibit.

CloudFormation template snippet:

Resources:
  MyRDSInstance:
    Type: AWS::RDS::DBInstance
    Properties:
      DBInstanceClass: db.t3.medium
      Engine: MySQL
      MultiAZ: true
      DBSubnetGroupName: !Ref MyDBSubnetGroup
      VPCSecurityGroups:
        - !Ref MyDBSecurityGroup
      StorageType: gp2
      AllocatedStorage: 100
      BackupRetentionPeriod: 7
      DeletionProtection: true

  MyDBSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for RDS
      VpcId: !Ref MyVPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 3306
          ToPort: 3306
          CidrIp: 10.0.0.0/16

  MyDBSubnetGroup:
    Type: AWS::RDS::DBSubnetGroup
    Properties:
      DBSubnetGroupDescription: Subnet group for RDS
      SubnetIds:
        - subnet-12345678
        - subnet-87654321

  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t2.micro
      ImageId: ami-0abcdef1234567890
      SecurityGroupIds:
        - !Ref MyEC2SecurityGroup
      SubnetId: subnet-12345678

  MyEC2SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Security group for EC2
      VpcId: !Ref MyVPC
      SecurityGroupIngress:
        - IpProtocol: tcp
          FromPort: 3306
          ToPort: 3306
          CidrIp: 0.0.0.0/0
        - IpProtocol: tcp
          FromPort: 22
          ToPort: 22
          CidrIp: 0.0.0.0/0

A
The EC2 security group allows inbound MySQL from 0.0.0.0/0 but the RDS security group only allows traffic from the VPC CIDR, which does not include the EC2 security group.
RDS SG should allow traffic from EC2 SG, not just VPC CIDR.
B
The RDS instance has encryption enabled, preventing access from EC2.
Why wrong: Encryption does not block network connectivity.
C
The RDS instance has DeletionProtection enabled, which blocks connections.
Why wrong: DeletionProtection prevents deletion, not connections.
D
The EC2 instance is in a different Availability Zone than the RDS instance.
Why wrong: Multi-AZ does not prevent connectivity across AZs.

Full breakdown with real-world context →

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 EC2 security group allows inbound MySQL from 0.0.0.0/0 but the RDS security group only allows traffic from the VPC CIDR, which does not include the EC2 security group.

Option A is correct because the RDS security group (MyDBSecurityGroup) only allows inbound traffic from the VPC CIDR (10.0.0.0/16), but the EC2 security group (MyEC2SecurityGroup) does not allow traffic from the RDS security group. Additionally, the EC2 instance is in the same VPC but the security group rules do not reference the EC2 security group. Option B is wrong because the subnets are in the same VPC and there is no mention of different AZs being an issue. Option C is wrong because there is no encryption configured. Option D is wrong because DeletionProtection does not affect connectivity.

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.

✓
The EC2 security group allows inbound MySQL from 0.0.0.0/0 but the RDS security group only allows traffic from the VPC CIDR, which does not include the EC2 security group.
Why this is correct
RDS SG should allow traffic from EC2 SG, not just VPC CIDR.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The RDS instance has encryption enabled, preventing access from EC2.
Why it's wrong here
Encryption does not block network connectivity.
✗
The RDS instance has DeletionProtection enabled, which blocks connections.
Why it's wrong here
DeletionProtection prevents deletion, not connections.
✗
The EC2 instance is in a different Availability Zone than the RDS instance.
Why it's wrong here
Multi-AZ does not prevent connectivity across AZs.

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 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.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DBS-C01 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Deployment and Migration — This question tests Deployment and Migration — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The EC2 security group allows inbound MySQL from 0.0.0.0/0 but the RDS security group only allows traffic from the VPC CIDR, which does not include the EC2 security group. — Option A is correct because the RDS security group (MyDBSecurityGroup) only allows inbound traffic from the VPC CIDR (10.0.0.0/16), but the EC2 security group (MyEC2SecurityGroup) does not allow traffic from the RDS security group. Additionally, the EC2 instance is in the same VPC but the security group rules do not reference the EC2 security group. Option B is wrong because the subnets are in the same VPC and there is no mention of different AZs being an issue. Option C is wrong because there is no encryption configured. Option D is wrong because DeletionProtection does not affect connectivity.

What should I do if I get this DBS-C01 question wrong?

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?

CIDR notation defines the prefix length.

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Last reviewed: Jun 20, 2026

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