How to avoid circular dependency between transit gateway and route table?

Question

In Cloudformation template I have an option to define `AssociationDefaultRouteTableId` and `PropagationDefaultRouteTableId` for TransitGateway.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-transitgateway.html

```
Type: AWS::EC2::TransitGateway
Properties: 
  ...
  AssociationDefaultRouteTableId: String
  ...
  PropagationDefaultRouteTableId: String
```

But TransitGatewayRouteTable has mandatory parameter `TransitGatewayId` and this causes a circular dependency between route table and transit gateway :-(

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-transitgatewayroutetable.html

```
Type: AWS::EC2::TransitGatewayRouteTable
Properties: 
  ...
  TransitGatewayId: String
```

Is there a way to get around this somehow and actually set `AssociationDefaultRouteTableId` and `PropagationDefaultRouteTableId` for transit gateway from Cloudformation template ?

Accepted Answer

I don't think you'll be able to do this how you are trying as the circular dependency will always be there.

One way that you may be able to achieve the same outcome is by using a custom lambda resource to modify the TGW defaults after it's created. Here is an example from github: https://github.com/alexandrespbr/vpcsharednfw/blob/3aad9e78c08610b2e3602364158f78867338db39/egressVpc.yaml#L475

How to avoid circular dependency between transit gateway and route table?

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