Amazon web services – How to switch Elastic IP from one EC2 instance to another using cloudformation?

Yes, it’s possible as outlined in the documentation: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/refactor-stacks.html

Here’s a demo (all of the code snippets below are run in bash).

Let’s say we have these two templates (note that the first template must have the deletion policy set to Retain for the EIP):

template1.yml :

AWSTemplateFormatVersion: 2010-09-09
Description:
  Testing stuff

Resources:
  myENI:
    Type: AWS::EC2::NetworkInterface
    Properties:
      SubnetId: <subnet-id>
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-02d7ab1717c7787ce
      InstanceType: t2.micro
      NetworkInterfaces:
        - NetworkInterfaceId: !Ref myENI
          DeviceIndex: '0'
      Tags:
        - Key: Name
          Value: test-instance-1
  myEIP:
    Type: AWS::EC2::EIP
    DeletionPolicy: Retain
    Properties:
      Domain: vpc
  EIPAssociation:
    Type: AWS::EC2::EIPAssociation
    Properties:
      AllocationId: !GetAtt myEIP.AllocationId
      NetworkInterfaceId: !Ref myENI
    DependsOn: Instance

template2.yml:

AWSTemplateFormatVersion: 2010-09-09
Description:
  Testing stuff

Resources:
  myENI:
    Type: AWS::EC2::NetworkInterface
    Properties:
      SubnetId: <subnet-id>
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-02d7ab1717c7787ce
      InstanceType: t2.micro
      NetworkInterfaces:
        - NetworkInterfaceId: !Ref myENI
          DeviceIndex: '0'
      Tags:
        - Key: Name
          Value: test-instance-2

then let’s deploy them both:

$ aws cloudformation deploy --template-file template1.yml --stack-name stack1
$ aws cloudformation deploy --template-file template2.yml --stack-name stack2

once the stacks have been deployed, we need to fetch the allocation id and the public ip of the elastic ip deployed in the first stack:

$ EIP_PHYSICAL_RESOURCE_ID=$(aws cloudformation describe-stack-resources --stack-name stack1 --query 'StackResources[?LogicalResourceId == `myEIP`].PhysicalResourceId' --output text)
$ EIP_ALLOCATION_ID=$(aws ec2 describe-addresses --query "Addresses[?PublicIp == '${EIP_PHYSICAL_RESOURCE_ID}'].AllocationId" --output text)

we can now proceed to remove the EIP and the EIP association from the first stack:

template1.yml

AWSTemplateFormatVersion: 2010-09-09
Description:
  Testing stuff

Resources:
  myENI:
    Type: AWS::EC2::NetworkInterface
    Properties:
      SubnetId: <subnet-id>
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-02d7ab1717c7787ce
      InstanceType: t2.micro
      NetworkInterfaces:
        - NetworkInterfaceId: !Ref myENI
          DeviceIndex: '0'
      Tags:
        - Key: Name
          Value: test-instance-1

and let’s update the first stack:

$ aws cloudformation update-stack --template-body file://template1.yml --stack-name stack1

now we must modify the second template to include the EIP.

template2.yml:

AWSTemplateFormatVersion: 2010-09-09
Description:
  Testing stuff

Resources:
  myENI:
    Type: AWS::EC2::NetworkInterface
    Properties:
      SubnetId: <subnet-id>
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-02d7ab1717c7787ce
      InstanceType: t2.micro
      NetworkInterfaces:
        - NetworkInterfaceId: !Ref myENI
          DeviceIndex: '0'
      Tags:
        - Key: Name
          Value: test-instance-2
  myEIP:
    Type: AWS::EC2::EIP
    DeletionPolicy: Retain
    Properties:
      Domain: vpc

now let’s create the following file (file.txt) which will be used in the import operation later:

[
  {
      "ResourceType":"AWS::EC2::EIP",
      "LogicalResourceId":"myEIP",
      "ResourceIdentifier": {
        "AllocationId": "$EIP_ALLOCATION_ID",
        "PublicIp": "$EIP_PHYSICAL_RESOURCE_ID"
      }
  }
]

and now let’s populate it (envsubst is a handy way to replace the two variables into the text file, however you can obviously do this in other ways too):

$ export EIP_PHYSICAL_RESOURCE_ID
$ export EIP_ALLOCATION_ID
$ envsubst < file.txt > resourcesToImport.txt

resourcesToImport.txt now contains the allocation id and public ip of the EIP.
Let’s run the import operation:

$ aws cloudformation create-change-set 
    --stack-name stack2 --change-set-name ImportChangeSet 
    --change-set-type IMPORT 
    --resources-to-import file://resourcesToImport.txt 
    --template-body file://template2.yml
$ aws cloudformation execute-change-set --change-set-name ImportChangeSet --stack-name stack2

the second stack now has imported the EIP. Finally, let’s create an association with the existing instance

template2.yml:

AWSTemplateFormatVersion: 2010-09-09
Description:
  Testing stuff

Resources:
  myENI:
    Type: AWS::EC2::NetworkInterface
    Properties:
      SubnetId: <subnet-id>
  Instance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: ami-02d7ab1717c7787ce
      InstanceType: t2.micro
      NetworkInterfaces:
        - NetworkInterfaceId: !Ref myENI
          DeviceIndex: '0'
      Tags:
        - Key: Name
          Value: test-instance-2
  myEIP:
    Type: AWS::EC2::EIP
    DeletionPolicy: Retain
    Properties:
      Domain: vpc
  EIPAssociation:
    Type: AWS::EC2::EIPAssociation
    Properties:
      AllocationId: !GetAtt myEIP.AllocationId
      NetworkInterfaceId: !Ref myENI
    DependsOn: Instance

and let’s update the stack:

$ aws cloudformation update-stack --template-body file://template2.yml --stack-name stack2

The EIP which was originally associated with the instance in the first stack is now associated with the instance in the second stack