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# AWS::EC2::TransitGatewayMulticastDomainAssociation

Associates the specified subnets and transit gateway attachments with the specified transit gateway multicast domain.
The transit gateway attachment must be in the available state before you can add a resource.
## Syntax

To declare this entity in your CloudFormation template, use the following syntax:
### JSON

```
{
 "Type" : "AWS::EC2::TransitGatewayMulticastDomainAssociation",
 "Properties" : {
 "[SubnetId](#cfn-ec2-transitgatewaymulticastdomainassociation-subnetid)" : {{String}},
 "[TransitGatewayAttachmentId](#cfn-ec2-transitgatewaymulticastdomainassociation-transitgatewayattachmentid)" : {{String}},
 "[TransitGatewayMulticastDomainId](#cfn-ec2-transitgatewaymulticastdomainassociation-transitgatewaymulticastdomainid)" : {{String}}
 }
}
```
### YAML

```
Type: AWS::EC2::TransitGatewayMulticastDomainAssociation
Properties:
 [SubnetId](#cfn-ec2-transitgatewaymulticastdomainassociation-subnetid): {{String}}
 [TransitGatewayAttachmentId](#cfn-ec2-transitgatewaymulticastdomainassociation-transitgatewayattachmentid): {{String}}
 [TransitGatewayMulticastDomainId](#cfn-ec2-transitgatewaymulticastdomainassociation-transitgatewaymulticastdomainid): {{String}}
```
## Properties

`SubnetId` 
The IDs of the subnets to associate with the transit gateway multicast domain. 
*Required*: Yes 
*Type*: String 
*Update requires*: [Replacement](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-update-behaviors.html#update-replacement)
`TransitGatewayAttachmentId` 
The ID of the transit gateway attachment. 
*Required*: Yes 
*Type*: String 
*Update requires*: [Replacement](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-update-behaviors.html#update-replacement)
`TransitGatewayMulticastDomainId` 
The ID of the transit gateway multicast domain. 
*Required*: Yes 
*Type*: String 
*Update requires*: [Replacement](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/using-cfn-updating-stacks-update-behaviors.html#update-replacement)
## Return values

### Ref

When you pass the logical ID of this resource to the intrinsic `Ref` function, `Ref` returns the transit gateway multicast domain ID. For example: `tgw-mcast-domain-000fb24d04EXAMPLE`.
### Fn::GetAtt

The `Fn::GetAtt` intrinsic function returns a value for a specified attribute of this type. The following are the available attributes and sample return values.
For more information about using the `Fn::GetAtt` intrinsic function, see [https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/intrinsic-function-reference-getatt.html](https://docs.aws.amazon.com/AWSCloudFormation/latest/TemplateReference/intrinsic-function-reference-getatt.html).
#### 

`ResourceId` 
The ID of the resource.
`ResourceType` 
The type of resource, for example a VPC attachment.
`State` 
The state of the resource.
## See also

+ [AssociateTransitGatewayMulticastDomain](https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_AssociateTransitGatewayMulticastDomain.html) in the *Amazon EC2 API Reference*
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