将现有的Elastic IP与CloudFormation脚本中的Ec2实例相关联？

Question

I have a CloudFormation stack that is used to create multiple copies of an environment. Some but not all of these environments will have elastic IPs associated.

Right now I:

1. Run the CF script
2. Assign the Elastic IP where approprite
3. On stack updates that don't touch the instance, Elastic IP is preserved to the instance which had it.

4. On updates to the instance itself,the association is lost.

   It kind of makes senses that when the underlying instance name is changed when updates are made from the CF stack itself, which causes the IP to become unassigned. So I would like to pass in the ID of the elastic IP as a parameter to the script and assign it manually.

It seems there are two ways to do this , but I don't understand the difference. What is an allocation Id, and when would you need to use it?

"IPAssoc" : {
     "Type" : "AWS::EC2::EIPAssociation",
     "Properties" : {
         "InstanceId" : { "Ref" : "logical name of an AWS::EC2::Instance resource" },
         "EIP" : "existing Elastic IP address"
     }
 }

vs

"VpcIPAssoc" : {
     "Type" : "AWS::EC2::EIPAssociation",
     "Properties" : {
         "InstanceId" : { "Ref" : "logical name of an AWS::EC2::Instance resource" },
         "AllocationId" : "existing VPC Elastic IP allocation ID"
     }
 }

Answer 1

The difference is between VPC usage and EC2 Classic.

As per http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-eip-association.html , EIP is for use with EC2 Classic and AllocationId is for VPC's.

For the difference between VPC and EC2 Classic, see http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-vpc.html#differences-ec2-classic-vpc .