How to Import an AWS Transit Gateway VPC Attachment into Terraform

Import AWS Transit Gateway VPC Attachment with Terraback (recommended)

Terraback reverse-engineers your live infrastructure: it reads the Amazon VPC resource with read-only credentials, generates the HCL, and produces the exact import block. Two commands take you from a live AWS Transit Gateway VPC Attachment to managed Terraform.

terraback scan all aws --region us-east-1

terraback aws import --method bulk

The Terraform import block

Terraback emits a Terraform 1.5+ import block like the one below. Because the block lives in your configuration, the import is reviewable in a pull request and repeatable across environments.

import {
  to = aws_ec2_transit_gateway_vpc_attachment.app
  id = "tgw-attach-0a1b2c3d4e5f6a7b8"
}

Example aws_ec2_transit_gateway_vpc_attachment configuration

Here is a realistic AWS Transit Gateway VPC Attachment block. Terraback generates a fuller version from your actual resource attributes; this is a minimal, valid starting point.

resource "aws_ec2_transit_gateway_vpc_attachment" "app" {
  transit_gateway_id = "tgw-0a1b2c3d4e5f6a7b8"
  vpc_id             = "vpc-0a1b2c3d4e5f6a7b8"
  subnet_ids         = ["subnet-0aa1", "subnet-0bb2"]
}

Gotchas when importing a AWS Transit Gateway VPC Attachment

• Import by the tgw-attach- ID, not the transit gateway ID or the VPC ID.
• This attachment is separate from the aws_ec2_transit_gateway itself and from any aws_ec2_transit_gateway_route_table_association or _propagation; import each piece you manage.
• subnet_ids should list one subnet per Availability Zone you want reachable; the attachment places an ENI in each, so adding or removing AZs is a real change.
• If default route table association and propagation are enabled on the transit gateway, the corresponding routes appear automatically and are not part of this resource.

Doing it manually with terraform import

The native approach is to write the aws_ec2_transit_gateway_vpc_attachment block by hand, then run terraform import aws_ec2_transit_gateway_vpc_attachment.example <import-id> for every resource, one at a time. That works for a handful of resources, but it does not scale: you author all the HCL yourself and repeat the command for each item. Terraback generates the HCL and the import blocks for your whole account in one pass.

Import other AWS resources