AWS CloudFormation VPC Infrastructure

Overview

Create production-ready VPC infrastructure using AWS CloudFormation templates. This skill covers VPC components (Subnets, Route Tables, NAT Gateways, Internet Gateways), template structure best practices, parameter patterns, and cross-stack references for modular, reusable infrastructure as code.

When to Use

Use this skill when:

Creating new VPCs with public and private subnets
Configuring route tables for internet and NAT connectivity
Setting up Internet Gateways and NAT Gateways
Implementing template Parameters with AWS-specific types
Creating Outputs for cross-stack references
Organizing templates with Mappings and Conditions
Designing reusable, modular CloudFormation templates

Instructions

Follow these steps to create VPC infrastructure with CloudFormation:

Define VPC Parameters: Specify CIDR block and DNS settings
Create Subnets: Configure public and private subnets across AZs
Set Up Internet Gateway: Enable internet connectivity for public subnets
Configure NAT Gateways: Provide outbound connectivity for private subnets
Create Route Tables: Define routing rules for each subnet type
Add Security Groups: Configure network access controls
Implement VPC Endpoints: Enable private connectivity to AWS services
Create VPC Peering: Connect multiple VPCs if needed

For complete examples, see the EXAMPLES.md file.

Examples

The following examples demonstrate common VPC patterns:

Example 1: VPC with Public and Private Subnets

VPC:
  Type: AWS::EC2::VPC
  Properties:
    CidrBlock: 10.0.0.0/16
    EnableDnsHostnames: true
    EnableDnsSupport: true

PublicSubnet:
  Type: AWS::EC2::Subnet
  Properties:
    VpcId: !Ref VPC
    CidrBlock: 10.0.1.0/24
    AvailabilityZone: !Select [0, !GetAZs ""]
    MapPublicIpOnLaunch: true

PrivateSubnet:
  Type: AWS::EC2::Subnet
  Properties:
    VpcId: !Ref VPC
    CidrBlock: 10.0.2.0/24
    AvailabilityZone: !Select [0, !GetAZs ""]

Example 2: Internet Gateway and Routes

InternetGateway:
  Type: AWS::EC2::InternetGateway

VPCGatewayAttachment:
  Type: AWS::EC2::VPCGatewayAttachment
  Properties:
    VpcId: !Ref VPC
    InternetGatewayId: !Ref InternetGateway

PublicRouteTable:
  Type: AWS::EC2::RouteTable
  Properties:
    VpcId: !Ref VPC

PublicRoute:
  Type: AWS::EC2::Route
  DependsOn: VPCGatewayAttachment
  Properties:
    RouteTableId: !Ref PublicRouteTable
    DestinationCidrBlock: 0.0.0.0/0
    GatewayId: !Ref InternetGateway

Example 3: NAT Gateway

NatGateway:
  Type: AWS::EC2::NatGateway
  Properties:
    AllocationId: !GetAtt EIP.AllocationId
    SubnetId: !Ref PublicSubnet

EIP:
  Type: AWS::EC2::EIP

PrivateRouteTable:
  Type: AWS::EC2::RouteTable
  Properties:
    VpcId: !Ref VPC

PrivateRoute:
  Type: AWS::EC2::Route
  Properties:
    RouteTableId: !Ref PrivateRouteTable
    DestinationCidrBlock: 0.0.0.0/0
    NatGatewayId: !Ref NatGateway

For complete production-ready examples, see EXAMPLES.md.

Quick Start

Basic VPC with Public Subnet

AWSTemplateFormatVersion: 2010-09-09
Description: Simple VPC with public subnet

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsHostnames: true
      EnableDnsSupport: true
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-vpc

  PublicSubnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.1.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-public-subnet

  InternetGateway:
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-igw

  VPCGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  PublicRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-public-rt

  DefaultPublicRoute:
    Type: AWS::EC2::Route
    DependsOn: VPCGatewayAttachment
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  PublicSubnetRouteTableAssociation:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet
      RouteTableId: !Ref PublicRouteTable

VPC with Public and Private Subnets

AWSTemplateFormatVersion: 2010-09-09
Description: VPC with public and private subnets across multiple AZs

Parameters:
  EnvironmentName:
    Type: String
    Default: production
    Description: Environment name for resource tagging

  VpcCidr:
    Type: String
    Default: 10.0.0.0/16
    Description: CIDR block for the VPC

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: !Ref VpcCidr
      EnableDnsHostnames: true
      EnableDnsSupport: true
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: Name
          Value: !Sub ${AWS::StackName}-vpc

  InternetGateway:
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: Name
          Value: !Sub ${AWS::StackName}-igw

  VPCGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  # Public Subnet 1
  PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.1.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: SubnetType
          Value: Public
        - Key: Name
          Value: !Sub ${AWS::StackName}-public-1

  PublicRouteTable1:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: Name
          Value: !Sub ${AWS::StackName}-public-rt-1

  DefaultPublicRoute1:
    Type: AWS::EC2::Route
    DependsOn: VPCGatewayAttachment
    Properties:
      RouteTableId: !Ref PublicRouteTable1
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  PublicSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet1
      RouteTableId: !Ref PublicRouteTable1

  # Private Subnet 1
  PrivateSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.10.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: SubnetType
          Value: Private
        - Key: Name
          Value: !Sub ${AWS::StackName}-private-1

  PrivateRouteTable1:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Environment
          Value: !Ref EnvironmentName
        - Key: Name
          Value: !Sub ${AWS::StackName}-private-rt-1

  PrivateSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PrivateSubnet1
      RouteTableId: !Ref PrivateRouteTable1

Outputs:
  VpcId:
    Description: VPC ID
    Value: !Ref VPC
    Export:
      Name: !Sub ${AWS::StackName}-VpcId

  PublicSubnet1Id:
    Description: Public Subnet 1 ID
    Value: !Ref PublicSubnet1
    Export:
      Name: !Sub ${AWS::StackName}-PublicSubnet1Id

  PrivateSubnet1Id:
    Description: Private Subnet 1 ID
    Value: !Ref PrivateSubnet1
    Export:
      Name: !Sub ${AWS::StackName}-PrivateSubnet1Id

Template Structure

Template Sections Overview

AWS CloudFormation templates are JSON or YAML files with specific sections. Each section serves a purpose in defining your infrastructure.

AWSTemplateFormatVersion: 2010-09-09  # Required - template version
Description: Optional description string  # Optional description

# Section order matters for readability but CloudFormation accepts any order
Mappings: {}       # Static configuration tables
Metadata: {}       # Additional information about resources
Parameters: {}     # Input values for customization
Rules: {}          # Parameter validation rules
Conditions: {}     # Conditional resource creation
Transform: {}      # Macro processing (e.g., AWS::Serverless)
Resources: {}      # AWS resources to create (REQUIRED)
Outputs: {}        # Return values after stack creation

Format Version

The AWSTemplateFormatVersion identifies the template version. Current version is 2010-09-09.

AWSTemplateFormatVersion: 2010-09-09
Description: My CloudFormation Template

Description

Add a description to document the template's purpose. Must appear after the format version.

AWSTemplateFormatVersion: 2010-09-09
Description: >
  This template creates a VPC with public and private subnets
  for hosting web applications. It includes:
  - Internet Gateway for public access
  - NAT Gateway for private subnet outbound access
  - Security groups for web and database tiers

Metadata

Use Metadata for additional information about resources or parameters.

Metadata:
  AWS::CloudFormation::Interface:
    ParameterGroups:
      - Label:
          default: Network Configuration
        Parameters:
          - VpcCidr
          - PublicSubnetCidr
          - PrivateSubnetCidr
      - Label:
          default: Tags
        Parameters:
          - EnvironmentName
          - ProjectName
    ParameterLabels:
      VpcCidr:
        default: VPC CIDR Block
      EnvironmentName:
        default: Environment Name

Resources Section

The Resources section is the only required section. It defines AWS resources to provision.

Resources:
  MyVPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsHostnames: true
      EnableDnsSupport: true
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-vpc

Parameters

Parameter Types

Use AWS-specific parameter types for validation and easier selection in the console.

Parameters:
  VpcId:
    Type: AWS::EC2::VPC::Id
    Description: Select an existing VPC

  SubnetId:
    Type: AWS::EC2::Subnet::Id
    Description: Select a subnet

  SecurityGroupIds:
    Type: List<AWS::EC2::SecurityGroup::Id>
    Description: Select existing security groups

  InstanceType:
    Type: AWS::EC2::InstanceType
    Description: EC2 instance type
    Default: t3.micro
    AllowedValues:
      - t3.micro
      - t3.small
      - t3.medium
      - t3.large

  AmiId:
    Type: AWS::EC2::Image::Id
    Description: Select an AMI

  KeyName:
    Type: AWS::EC2::KeyPair::KeyName
    Description: Select an existing key pair

Parameter Constraints

Add constraints to validate parameter values.

Parameters:
  VpcCidr:
    Type: String
    Description: CIDR block for the VPC
    Default: 10.0.0.0/16
    AllowedPattern: ^([0-9]{1,3}\.){3}[0-9]{1,3}/[0-9]{1,2}$
    ConstraintDescription: Must be a valid CIDR block (x.x.x.x/x)

  InstanceCount:
    Type: Number
    Description: Number of instances to launch
    Default: 1
    MinValue: 1
    MaxValue: 10

  Environment:
    Type: String
    Description: Deployment environment
    Default: development
    AllowedValues:
      - development
      - staging
      - production
    ConstraintDescription: Must be development, staging, or production

SSM Parameter Types

Reference Systems Manager parameters for dynamic values.

Parameters:
  LatestAmiId:
    Type: AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>
    Description: Latest AMI ID from SSM
    Default: /aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2

Mappings

Use Mappings for static configuration data based on regions or other factors.

Mappings:
  RegionMap:
    us-east-1:
      HVM64: ami-0ff8a95407f89df2f
      HVMG2: ami-0a0c776d80e2a1f3c
    us-west-2:
      HVM64: ami-0a0c776d80e2a1f3c
      HVMG2: ami-0a0c776d80e2a1f3c
    eu-west-1:
      HVM64: ami-0ff8a95407f89df2f
      HVMG2: ami-0a0c776d80e2a1f3c

  EnvironmentConfig:
    development:
      InstanceType: t3.micro
      MinInstances: 1
      MaxInstances: 2
    staging:
      InstanceType: t3.small
      MinInstances: 1
      MaxInstances: 3
    production:
      InstanceType: t3.medium
      MinInstances: 2
      MaxInstances: 10

Resources:
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: !FindInMap [RegionMap, !Ref AWS::Region, HVM64]
      InstanceType: !FindInMap [EnvironmentConfig, !Ref Environment, InstanceType]

Conditions

Use Conditions to conditionally create resources based on parameters.

Parameters:
  DeployNatGateway:
    Type: String
    Default: true
    AllowedValues:
      - true
      - false

  Environment:
    Type: String
    Default: development
    AllowedValues:
      - development
      - staging
      - production

Conditions:
  ShouldDeployNat: !Equals [!Ref DeployNatGateway, true]
  IsProduction: !Equals [!Ref Environment, production]

Resources:
  NatGateway:
    Type: AWS::EC2::NatGateway
    Condition: ShouldDeployNat
    Properties:
      AllocationId: !If
        - ShouldDeployNat
        - !GetAtt EIP.AllocationId
        - !Ref AWS::NoValue
      SubnetId: !Ref PublicSubnet

  ProductionOnlyResource:
    Type: AWS::EC2::VPCEndpoint
    Condition: IsProduction
    Properties:
      ServiceName: !Sub com.amazonaws.${AWS::Region}.s3
      VpcId: !Ref VPC

Transform

Use Transform for macros like AWS::Serverless for SAM templates.

AWSTemplateFormatVersion: 2010-09-09
Transform: AWS::Serverless-2016-10-31
Description: SAM template for serverless application

Resources:
  MyFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: index.handler
      Runtime: nodejs18.x
      CodeUri: function/
      Events:
        ApiEvent:
          Type: Api
          Properties:
            Path: /{proxy+}
            Method: ANY

Outputs and Cross-Stack References

Basic Outputs

Outputs:
  VpcId:
    Description: VPC ID
    Value: !Ref VPC

  PublicSubnetId:
    Description: Public Subnet ID
    Value: !Ref PublicSubnet

  VpcCidr:
    Description: VPC CIDR Block
    Value: !GetAtt VPC.CidrBlock

Exporting Values for Cross-Stack References

Export values so other stacks can import them.

Outputs:
  VpcId:
    Description: VPC ID for other stacks
    Value: !Ref VPC
    Export:
      Name: !Sub ${AWS::StackName}-VpcId

  PublicSubnetIds:
    Description: Comma-separated public subnet IDs
    Value: !Join [",", [!Ref PublicSubnet1, !Ref PublicSubnet2]]
    Export:
      Name: !Sub ${AWS::StackName}-PublicSubnetIds

  PrivateSubnetIds:
    Description: Comma-separated private subnet IDs
    Value: !Join [",", [!Ref PrivateSubnet1, !Ref PrivateSubnet2]]
    Export:
      Name: !Sub ${AWS::StackName}-PrivateSubnetIds

Importing Values in Another Stack

Parameters:
  VpcId:
    Type: AWS::EC2::VPC::Id
    Description: VPC ID from network stack
    # User selects from exported values in console

  # Or use Fn::ImportValue for programmatic access
Resources:
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      VpcId: !ImportValue
        Fn::Sub: ${NetworkStackName}-VpcId
      GroupDescription: Security group for application

Cross-Stack Reference Pattern

Create a dedicated network stack that exports values:

# network-stack.yaml
AWSTemplateFormatVersion: 2010-09-09
Description: Network infrastructure stack

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-vpc

  PublicSubnets:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.1.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      MapPublicIpOnLaunch: true

Outputs:
  VpcId:
    Value: !Ref VPC
    Export:
      Name: !Sub ${AWS::StackName}-VpcId

  PublicSubnetIds:
    Value: !Ref PublicSubnets
    Export:
      Name: !Sub ${AWS::StackName}-PublicSubnetIds

Application stack imports these values:

# application-stack.yaml
AWSTemplateFormatVersion: 2010-09-09
Description: Application stack that imports from network

Parameters:
  NetworkStackName:
    Type: String
    Description: Name of the network stack
    Default: network-stack

Resources:
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      SubnetId: !ImportValue
        Fn::Sub: ${NetworkStackName}-PublicSubnetIds
      InstanceType: t3.micro

VPC Components

VPC with All Components

AWSTemplateFormatVersion: 2010-09-09
Description: Complete VPC with public/private subnets, NAT, and IGW

Parameters:
  EnvironmentName:
    Type: String
    Default: production

Resources:
  # VPC
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsHostnames: true
      EnableDnsSupport: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-vpc
        - Key: Environment
          Value: !Ref EnvironmentName

  # Internet Gateway
  InternetGateway:
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-igw
        - Key: Environment
          Value: !Ref EnvironmentName

  InternetGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  # NAT Gateway (with EIP)
  NatGatewayEIP:
    Type: AWS::EC2::EIP
    DependsOn: InternetGatewayAttachment
    Properties:
      Domain: vpc
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-nat-eip

  NatGateway:
    Type: AWS::EC2::NatGateway
    Properties:
      AllocationId: !GetAtt NatGatewayEIP.AllocationId
      SubnetId: !Ref PublicSubnet1
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-nat-gw
        - Key: Environment
          Value: !Ref EnvironmentName

  # Public Subnets
  PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.1.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-public-1
        - Key: SubnetType
          Value: Public

  PublicSubnet2:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.2.0/24
      AvailabilityZone: !Select [1, !GetAZs '']
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-public-2
        - Key: SubnetType
          Value: Public

  # Private Subnets
  PrivateSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.10.0/24
      AvailabilityZone: !Select [0, !GetAZs '']
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-private-1
        - Key: SubnetType
          Value: Private

  PrivateSubnet2:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.11.0/24
      AvailabilityZone: !Select [1, !GetAZs '']
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-private-2
        - Key: SubnetType
          Value: Private

  # Public Route Table
  PublicRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-public-rt

  DefaultPublicRoute:
    Type: AWS::EC2::Route
    DependsOn: InternetGatewayAttachment
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  PublicSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet1
      RouteTableId: !Ref PublicRouteTable

  PublicSubnetRouteTableAssociation2:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet2
      RouteTableId: !Ref PublicRouteTable

  # Private Route Table (with NAT)
  PrivateRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-private-rt

  DefaultPrivateRoute:
    Type: AWS::EC2::Route
    Properties:
      RouteTableId: !Ref PrivateRouteTable
      DestinationCidrBlock: 0.0.0.0/0
      NatGatewayId: !Ref NatGateway

  PrivateSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PrivateSubnet1
      RouteTableId: !Ref PrivateRouteTable

  PrivateSubnetRouteTableAssociation2:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PrivateSubnet2
      RouteTableId: !Ref PrivateRouteTable

Outputs:
  VpcId:
    Value: !Ref VPC
    Export:
      Name: !Sub ${EnvironmentName}-VpcId

  InternetGatewayId:
    Value: !Ref InternetGateway
    Export:
      Name: !Sub ${EnvironmentName}-InternetGatewayId

  PublicSubnetIds:
    Value: !Join [",", [!Ref PublicSubnet1, !Ref PublicSubnet2]]
    Export:
      Name: !Sub ${EnvironmentName}-PublicSubnetIds

  PrivateSubnetIds:
    Value: !Join [",", [!Ref PrivateSubnet1, !Ref PrivateSubnet2]]
    Export:
      Name: !Sub ${EnvironmentName}-PrivateSubnetIds

  NatGatewayId:
    Value: !Ref NatGateway
    Export:
      Name: !Sub ${EnvironmentName}-NatGatewayId

Best Practices

Use AWS-Specific Parameter Types

Always use AWS-specific parameter types for validation and easier selection.

Parameters:
  VpcId:
    Type: AWS::EC2::VPC::Id
    Description: Select a VPC

  SubnetIds:
    Type: List<AWS::EC2::Subnet::Id>
    Description: Select subnets

  SecurityGroupIds:
    Type: List<AWS::EC2::SecurityGroup::Id>
    Description: Select security groups

Organize by Lifecycle

Separate resources that change at different rates into different stacks.

# Network stack - rarely changes
AWSTemplateFormatVersion: 2010-09-09
Description: Network infrastructure (VPC, subnets, routes)
Resources:
  VPC: AWS::EC2::VPC
  Subnets: AWS::EC2::Subnet
  RouteTables: AWS::EC2::RouteTable

# Application stack - changes frequently
AWSTemplateFormatVersion: 2010-09-09
Description: Application resources
Parameters:
  NetworkStackName:
    Type: String
Resources:
  Instances: AWS::EC2::Instance

Use Meaningful Names

Use AWS::StackName and parameters for consistent naming.

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      Tags:
        - Key: Name
          Value: !Sub ${AWS::StackName}-vpc
        - Key: Environment
          Value: !Ref EnvironmentName

Use Pseudo Parameters

Use pseudo parameters for region-agnostic templates.

Resources:
  S3Bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub ${AWS::StackName}-${AWS::AccountId}-${AWS::Region}

  LambdaFunction:
    Type: AWS::Lambda::Function
    Properties:
      Runtime: nodejs18.x
      Handler: index.handler
      Role: !GetAtt LambdaExecutionRole.Arn

Validate Before Deployment

# Validate template
aws cloudformation validate-template --template-body file://template.yaml

# Check for syntax errors
aws cloudformation validate-template \
  --template-body file://template.yaml \
  --query 'Description'

# Use cfn-lint for advanced validation
pip install cfn-lint
cfn-lint template.yaml

Stack Policies

Stack Policies prevent unintentional updates to critical resources. Use them to protect production infrastructure.

{
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "Update:*",
      "Principal": "*",
      "Resource": "*"
    },
    {
      "Effect": "Deny",
      "Action": ["Update:Replace", "Update:Delete"],
      "Principal": "*",
      "Resource": "LogicalId=ProductionDatabase"
    },
    {
      "Effect": "Deny",
      "Action": "Update:Replace",
      "Principal": "*",
      "Resource": "LogicalId=VPC"
    }
  ]
}

Apply the stack policy:

aws cloudformation set-stack-policy \
  --stack-name my-production-stack \
  --stack-policy-body file://stack-policy.json

Common Stack Policy Use Cases:

Protect database resources: Prevent accidental replacement of RDS instances
Protect VPC infrastructure: Prevent changes that could disrupt connectivity
Protect IAM roles: Prevent modifications that could break authorization

Termination Protection

Enable termination protection to prevent accidental stack deletion. This is critical for production environments.

Enable via AWS Console:

Go to CloudFormation > Stacks
Select your stack
Click "Stack actions" > "Enable termination protection"

Enable via AWS CLI:

# Enable termination protection
aws cloudformation update-termination-protection \
  --stack-name my-production-stack \
  --enable-termination-protection

# Disable termination protection (requires console access)
aws cloudformation update-termination-protection \
  --stack-name my-production-stack \
  --no-enable-termination-protection

Enable via CloudFormation (for new stacks):

Resources:
  ProductionStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/my-bucket/production.yaml
      TerminationProtection: true

Important Considerations:

Termination protection does not prevent stack updates
To delete a protected stack, you must first disable termination protection
Nested stacks inherit termination protection from parent stacks
Always enable for production and staging environments

Drift Detection

Drift detection identifies differences between your CloudFormation stack and the actual infrastructure. Run regular drift checks to ensure compliance.

Detect Drift on a Stack:

# Detect drift on a stack
aws cloudformation detect-drift \
  --stack-name my-vpc-stack

# Check drift status
aws cloudformation describe-stack-drift-detection-status \
  --stack-drift-detection-id <detection-id>

# Get drift detection results
aws cloudformation describe-stack-resource-drifts \
  --stack-name my-vpc-stack

Drift Status Values:

IN_SYNC: Resource matches the template
DRIFTED: Resource has been modified outside CloudFormation
NOT_CHECKED: Resource was not checked
UNKNOWN: Drift status could not be determined

Automated Drift Detection with Events:

# Use AWS Config for continuous drift monitoring
Resources:
  ConfigRule:
    Type: AWS::Config::ConfigRule
    Properties:
      ConfigRuleName: cloudformation-drift-detection
      Scope:
        ComplianceResourceTypes:
          - AWS::EC2::VPC
          - AWS::EC2::Subnet
          - AWS::EC2::SecurityGroup
      Source:
        Owner: CUSTOM_LAMBDA
        SourceIdentifier:
          Fn::GetAtt: [DriftDetectionFunction, Arn]

Best Practices for Drift Detection:

Run drift detection weekly for production stacks
Set up CloudWatch Events to trigger drift detection on schedule
Document and address all drift immediately
Use drift detection as part of change management process

Change Sets

Change Sets allow you to preview stack changes before applying them. This is essential for production deployments.

Create and Review a Change Set:

# Create a change set
aws cloudformation create-change-set \
  --stack-name my-vpc-stack \
  --template-body file://updated-template.yaml \
  --change-set-name vpc-update-changeset \
  --capabilities CAPABILITY_IAM

# List change sets
aws cloudformation list-change-sets \
  --stack-name my-vpc-stack

# Describe change set
aws cloudformation describe-change-set \
  --stack-name my-vpc-stack \
  --change-set-name vpc-update-changeset

# Execute change set
aws cloudformation execute-change-set \
  --stack-name my-vpc-stack \
  --change-set-name vpc-update-changeset

# Delete change set (if not executing)
aws cloudformation delete-change-set \
  --stack-name my-vpc-stack \
  --change-set-name vpc-update-changeset

Change Set Types:

Type	Description	Use Case
`UPDATE`	Standard update	Regular changes
`CREATE`	Creates new stack	Initial deployment
`IMPORT`	Imports existing resources	Lift-and-shift

Change Set Output Example:

{
  "ChangeSetId": "arn:aws:cloudformation:us-east-1:123456789:changeSet/...",
  "Changes": [
    {
      "Type": "Resource",
      "ResourceChange": {
        "Action": "Modify",
        "LogicalResourceId": "VPC",
        "PhysicalResourceId": "vpc-12345678",
        "Replacement": "False",
        "Details": [
          {
            "Target": {
              "Attribute": "Tags",
              "RequiresRecreation": "Never"
            },
            "Evaluation": "Static",
            "ChangeSource": "DirectModification"
          }
        ]
      }
    }
  ],
  "ExecutionStatus": "AVAILABLE",
  "Status": "CREATE_COMPLETE"
}

Best Practices for Change Sets:

Always create a change set before updating production stacks
Review all changes carefully before execution
Use meaningful change set names (e.g., vpc-security-update-2024-01)
Execute change sets promptly after review
Set a TTL on change sets if your organization requires approval workflows

CI/CD Integration with Change Sets

# GitHub Actions workflow for safe deployments
name: Deploy CloudFormation
on:
  push:
    branches: [main]

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Configure AWS Credentials
        uses: aws-actions/configure-aws-credentials@v4
        with:
          role-to-assume: arn:aws:iam::123456789:role/GitHubActionsRole
          aws-region: us-east-1

      - name: Create Change Set
        id: changeset
        run: |
          aws cloudformation create-change-set \
            --stack-name ${{ env.STACK_NAME }} \
            --template-body file://template.yaml \
            --change-set-name preview-changes \
            --capabilities CAPABILITY_IAM \
            --query 'Id' \
            --output text

      - name: Describe Change Set
        run: |
          aws cloudformation describe-change-set \
            --stack-name ${{ env.STACK_NAME }} \
            --change-set-name preview-changes

      - name: Execute Change Set (Manual approval required)
        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
        run: |
          aws cloudformation execute-change-set \
            --stack-name ${{ env.STACK_NAME }} \
            --change-set-name preview-changes

Related Resources

For advanced patterns: See EXAMPLES.md
For reference: See REFERENCE.md
AWS CloudFormation User Guide: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/
AWS VPC Documentation: https://docs.aws.amazon.com/vpc/latest/userguide/

Constraints and Warnings

Resource Limits

VPC Limits: Maximum 5 VPCs per region (soft limit, can be increased)
Subnet Limits: Maximum 200 subnets per VPC
Route Table Limits: Maximum 200 route tables per VPC
Internet Gateway: One Internet Gateway per VPC
NAT Gateway: One NAT Gateway per availability zone

Network Constraints

CIDR Overlap: VPC CIDR blocks cannot overlap with peered VPCs or connected networks
CIDR Changes: VPC CIDR blocks cannot be modified after creation; recreation required
Subnet Sizing: Subnet CIDR ranges cannot be changed after subnet creation
Route Propagation: Route propagation affects all routes in the route table

Security Constraints

Security Group Limits: Maximum 60 inbound and 60 outbound rules per security group
NACL Stateless: Network ACLs are stateless; return traffic must be explicitly allowed
Flow Logs Costs: VPC Flow Logs can generate significant CloudWatch Logs costs
Security Group References: Security group references cannot span VPC peering in some configurations

Operational Constraints

Subnet Isolation: Subnets cannot be moved between AZs after creation
Gateway Attachments: attaching/detaching Internet Gateways and VPGs takes time
Peering Limitations: VPC peering has limitations with transitive routing
VPN Connections: Site-to-Site VPN connections have bandwidth limitations

Cost Considerations

NAT Gateway: NAT gateways incur hourly costs plus data processing per GB
Transit Gateway: Transit Gateway attachments incur per-AZ hourly and data transfer costs
PrivateLink: Interface VPC endpoints incur hourly and per-GB data processing costs
Traffic Mirroring: Traffic mirroring doubles network costs

Availability Constraints

Multi-AZ Requirements: Resources should be distributed across multiple AZs for HA
AZ Isolation: Entire AZ failures can affect applications not distributed across AZs
Gateway Redundancy: Internet Gateways and NAT Gateways are single points of failure within AZ
Route Table Limits: Associate each subnet with only one route table