How to Import an AWS Transit Gateway Route Table Association into Terraform

Import AWS Transit Gateway Route Table Association with Terraback (recommended)

Terraback reverse-engineers your live infrastructure: it reads the Amazon VPC Transit Gateway resource with read-only credentials, generates the HCL, and produces the exact import block. Two commands take you from a live AWS Transit Gateway Route Table Association 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_route_table_association.example
  id = "tgw-rtb-12345678_tgw-attach-87654321"
}

Example aws_ec2_transit_gateway_route_table_association configuration

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

resource "aws_ec2_transit_gateway_route_table_association" "example" {
  transit_gateway_attachment_id  = "tgw-attach-87654321"
  transit_gateway_route_table_id = "tgw-rtb-12345678"
}

Gotchas when importing a AWS Transit Gateway Route Table Association

• This binds an attachment to a route table; the composite import ID joins the route table ID and the attachment ID with an underscore, and both parents must already exist in state.
• An attachment can be associated with only one route table at a time, so importing this when the attachment is still on the default association route table will produce a move, not a clean import.
• Do not confuse association with propagation: association controls which route table an attachment uses for routing decisions, while aws_ec2_transit_gateway_route_table_propagation controls which routes get advertised.
• Both transit_gateway_attachment_id and transit_gateway_route_table_id are force-new; changing either after import re-associates the attachment, which can briefly disrupt traffic.

Doing it manually with terraform import

The native approach is to write the aws_ec2_transit_gateway_route_table_association block by hand, then run terraform import aws_ec2_transit_gateway_route_table_association.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