Terraform AWS Transit-gateway

This module creates a Transit Gateway (TGW) and allows you to attach multiple VPCs with customizable route propagation and association settings.

Architecture

Providers

Name	Version
aws	5.82.2
Terraform	>= 1.12.1

Usage

module "transit_gateway" {
 source = "OT-CLOUD-KIT/terraform-aws-rds-postgres"

 amazon_side_asn                    = 64512
 auto_accept_shared_attachments     = "enable"
 default_route_table_association    = "enable"
 default_route_table_propagation    = "enable"
 dns_support                        = "enable"
 multicast_support                  = "disable"
 vpn_ecmp_support                   = "enable"
 security_group_referencing_support = "disable"
 transit_gateway_cidr_blocks        = ["172.31.0.0/16"]

 vpc_attachments = {
   "vpc-a" = {
     vpc_id                             = "vpc-0123456789abcdef0"
     subnet_ids                         = ["subnet-03b5635832hg", "subnet-5h673490bg"]
     dns_support                        = true
     ipv6_support                       = false
     appliance_mode_support             = false
     security_group_referencing_support = false
     default_association                = true
     default_propagation                = true
   }
   "vpc-b" = {
     vpc_id                             = "vpc-0abcdef1234567890"
     subnet_ids                         = ["subnet-bbb11146fga1", "subnet-bbbaQFG222"]
     dns_support                        = true
     ipv6_support                       = false
     appliance_mode_support             = false
     security_group_referencing_support = false
     default_association                = true
     default_propagation                = true
   }
 }

 tgw_route_cidr_block = {
   "vpc-a" = {
     cidr           = "10.0.0.0/16"
     attachment_key = "vpc-a"
   }
   "vpc-b" = {
     cidr           = "10.1.0.0/16"
     attachment_key = "vpc-b"
   }
 }

 tags = {
   Environment = "dev"
   Owner       = "team-x"
 }

 transit_gateway_name = "tgw"
}

Note:
The above example demonstrates how to use the module. All variables, resources, and outputs used here are already defined within this module.

Related Module:
If you're looking for a production-ready VPC setup with best practices (CIDR structure, subnets, route tables, flow logs, tagging, etc.), check out our
Terraform AWS Network Skeleton

Resources

Name	Type
aws_ec2_transit_gateway	resource
aws_ec2_transit_gateway_vpc_attachment	resource
aws_route	resource

---

Inputs

Name	Description	Type	Default	Required
amazon_side_asn	ASN for the Amazon side of the TGW	number	4200000000	no
auto_accept_shared_attachments	Automatically accept shared attachments	string	"enable"	no
default_route_table_association	Automatically associate TGW route tables	string	"enable"	no
default_route_table_propagation	Automatically propagate to TGW route tables	string	"enable"	no
dns_support	Enable DNS support for TGW	string	"enable"	no
transit_gateway_cidr_blocks	List of CIDR blocks assigned to the TGW	list(string)	["10.200.0.0/16"]	no
transit_gateway_name	Name tag for the TGW	string	"prod-tgw"	no
tags	Common tags applied to all TGW resources	map(string)	{ Environment = "prod", Owner = "Nikita" }	no
tgw_route_cidr_block	Global CIDR block for adding TGW routes	string	"10.0.0.0/8"	yes
vpc_attachments	List of VPC attachments with details	list(object)	[]	yes
name	Name of the TGW VPC attachment	string	tgw-attachment-vpc-a	yes
vpc_id	VPC ID to attach to the TGW	string	vpc-0b2e7e2387bf08301	yes
subnet_ids	List of subnet IDs used for TGW attachment (must be in different AZs)	list(string)	-	yes
route_table_id	Route table ID to which TGW routes will be added	string	-	yes
dns_support	Enable or disable DNS support for attachment	string	-	yes
ipv6_support	Enable or disable IPv6 support	string	-	yes
associate_with_tgw_route_table	Whether to associate this attachment with the TGW route table	bool	-	yes
propagate_to_tgw_route_table	Whether to propagate routes to the TGW route table	bool	-	yes

Outputs

Name	Description
transit_gateway_id	The ID of the created Transit Gateway
vpc_attachment_ids	A map of VPC attachment names to their TGW attachment IDs

---

Considerations

• Ensure VPCs, subnets, and route tables exist before applying this module.
• Validate that subnet IDs are correct and belong to the specified VPC.

Contributors

• Piyush Upadhyay
• Nikita Joshi