hub

This component is responsible for provisioning an AWS Transit Gateway hub that acts as a centralized gateway for connecting VPCs from other spoke accounts.

Usage

Stack Level: Regional

Basic Usage with tgw/spoke

Here's an example snippet for how to configure and use this component:

components:
  terraform:
    tgw/hub/defaults:
      metadata:
        type: abstract
        component: tgw/hub
      vars:
        enabled: true
        name: tgw-hub
        expose_eks_sg: false
        tags:
          Team: sre
          Service: tgw-hub

    tgw/hub:
      metadata:
        inherits:
          - tgw/hub/defaults
        component: tgw/hub
      vars:
        connections:
          - account:
              tenant: core
              stage: network
            vpc_component_names:
              - vpc-dev
          - account:
              tenant: core
              stage: artifacts
          - account:
              tenant: core
              stage: auto
            eks_component_names:
              - eks/cluster
          - account:
              tenant: plat
              stage: dev
            vpc_component_names:
              - vpc
              - vpc/data/1
            eks_component_names:
              - eks/cluster
          - account:
              tenant: plat
              stage: staging
            vpc_component_names:
              - vpc
              - vpc/data/1
            eks_component_names:
              - eks/cluster
          - account:
              tenant: plat
              stage: prod
            vpc_component_names:
              - vpc
              - vpc/data/1
            eks_component_names:
              - eks/cluster

To provision the Transit Gateway and all related resources, run the following commands:

atmos terraform plan tgw/hub -s <tenant>-<environment>-network
atmos terraform apply tgw/hub -s <tenant>-<environment>-network

Alternate Usage with tgw/attachment, tgw/routes, and vpc/routes

Components Overview

• tgw/hub: Creates the Transit Gateway in the network account
• tgw/attachment: Creates and manages Transit Gateway VPC attachments in connected accounts
• tgw/hub-connection: Creates the Transit Gateway peering connection between two tgw/hub deployments
• tgw/routes: Manages Transit Gateway route tables in the network account
• vpc-routes (vpc/routes/private): Configures VPC route tables in connected accounts to route traffic through the Transit Gateway (Note: This component lives outside the tgw/ directory since it's not specific to Transit Gateway)

Architecture

The Transit Gateway components work together in the following way:

1. Transit Gateway is created in the network account (tgw/hub)
2. VPCs in other accounts attach to the Transit Gateway (tgw/attachment)
3. Route tables in connected VPCs direct traffic across accounts (vpc-routes)
4. Transit Gateway route tables control routing between attachments (tgw/routes)

Deployment Steps

1. Deploy Transit Gateway Hub

First, create the Transit Gateway in the network account.

tip

Leave var.connections empty. With this refactor, the tgw/hub component is only responsible for creating the Transit Gateway and its route tables. We do not need to fetch and store outputs for the connected components anymore.

components:
  terraform:
    tgw/hub:
      vars:
        connections: []

2. Deploy VPC Attachments

Important: Deploy attachments in connected accounts first, before deploying attachments in the network account.

Connected Account Attachments

components:
  terraform:
    tgw/attachment:
      vars:
        transit_gateway_id: !terraform.output tgw/hub core-use1-network transit_gateway_id
        transit_gateway_route_table_id: !terraform.output tgw/hub core-use1-network transit_gateway_route_table_id
        create_transit_gateway_route_table_association: false

Network Account Attachment

components:
  terraform:
    tgw/attachment:
      vars:
        transit_gateway_id: !terraform.output tgw/hub core-use1-network transit_gateway_id
        transit_gateway_route_table_id: !terraform.output tgw/hub core-use1-network transit_gateway_route_table_id

        # Route table associations are required so that route tables can propagate their routes to other route tables.
        # Set the following to true in the same account where the Transit Gateway and its route tables are deployed
        create_transit_gateway_route_table_association: true

        # Associate connected accounts with the Transit Gateway route table
        additional_associations:
          - attachment_id: !terraform.output tgw/attachment core-use1-auto transit_gateway_vpc_attachment_id
            route_table_id: !terraform.output tgw/hub transit_gateway_route_table_id
          - attachment_id: !terraform.output tgw/attachment plat-use1-dev transit_gateway_vpc_attachment_id
            route_table_id: !terraform.output tgw/hub transit_gateway_route_table_id

3. Configure VPC Routes

Configure routes in all connected VPCs.

components:
  terraform:
    vpc/routes/private:
      metadata:
        component: vpc-routes
      vars:
        route_table_ids: !terraform.output vpc private_route_table_ids
        routes:
          # Route to network account
          - destination:
              cidr_block: !terraform.output vpc core-use1-network vpc_cidr
            target:
              type: transit_gateway_id
              value: !terraform.output tgw/hub core-use1-network transit_gateway_id

          # Route to core-auto account, if necessary
          - destination:
              cidr_block: !terraform.output vpc core-use1-auto vpc_cidr
            target:
              type: transit_gateway_id
              value: !terraform.output tgw/hub core-use1-network transit_gateway_id

Configure routes in the Network Account VPCs.

components:
  terraform:
    vpc/routes/private:
      vars:
        route_table_ids: !terraform.output vpc private_route_table_ids
        routes:
          # Routes to connected accounts
          - destination:
              cidr_block: !terraform.output vpc core-use1-auto vpc_cidr
            target:
              type: transit_gateway_id
              value: !terraform.output tgw/hub transit_gateway_id
          - destination:
              cidr_block: !terraform.output vpc plat-use1-dev vpc_cidr
            target:
              type: transit_gateway_id
              value: !terraform.output tgw/hub transit_gateway_id

4. Deploy Transit Gateway Route Table Routes

Deploy the tgw/routes component in the network account to create route tables and routes.

components:
  terraform:
    tgw/routes:
      vars:
        transit_gateway_route_table_id: !terraform.output tgw/hub transit_gateway_route_table_id
        # Use propagated routes to route through VPC attachments
        propagated_routes:
          # Route to this account
          - attachment_id: !terraform.output tgw/attachment core-use1-network transit_gateway_attachment_id
          # Route to any connected account
          - attachment_id: !terraform.output tgw/attachment core-use1-auto transit_gateway_attachment_id
          - attachment_id: !terraform.output tgw/attachment plat-use1-dev transit_gateway_attachment_id

Variables

Required Variables

region (string) required

AWS Region

Optional Variables

account_map_environment_name (string) optional

The name of the environment where account_map is provisioned

Default value: "gbl"

account_map_stage_name (string) optional

The name of the stage where account_map is provisioned

Default value: "root"

account_map_tenant_name (string) optional

The name of the tenant where account_map is provisioned.

If the tenant label is not used, leave this as null.

Default value: null

allow_external_principals (bool) optional

Set true to allow the TGW to be RAM shared with external principals specified in ram_principals

Default value: false

connections optional

A list of objects to define each TGW connections.

By default, each connection will look for only the default vpc component.

Type:

list(object({
account = object({
  stage       = string
  environment = optional(string, "")
  tenant      = optional(string, "")
})
vpc_component_names = optional(list(string), ["vpc"])
eks_component_names = optional(list(string), [])
}))

Default value: [ ]

expose_eks_sg (bool) optional

Set true to allow EKS clusters to accept traffic from source accounts

Default value: true

ram_principals (list(string)) optional

A list of AWS account IDs to share the TGW with outside the organization

Default value: [ ]

ram_resource_share_enabled (bool) optional

Enable Resource Access Managment Share

Default value: true

Context Variables

The following variables are defined in the context.tf file of this module and part of the terraform-null-label pattern.

additional_tag_map (map(string)) optional

Additional key-value pairs to add to each map in tags_as_list_of_maps. Not added to tags or id.
This is for some rare cases where resources want additional configuration of tags
and therefore take a list of maps with tag key, value, and additional configuration.

Required: No

Default value: { }

attributes (list(string)) optional

ID element. Additional attributes (e.g. workers or cluster) to add to id,
in the order they appear in the list. New attributes are appended to the
end of the list. The elements of the list are joined by the delimiter
and treated as a single ID element.

Required: No

Default value: [ ]

context (any) optional

Single object for setting entire context at once.
See description of individual variables for details.
Leave string and numeric variables as null to use default value.
Individual variable settings (non-null) override settings in context object,
except for attributes, tags, and additional_tag_map, which are merged.

Required: No

Default value:

{
  "additional_tag_map": {},
  "attributes": [],
  "delimiter": null,
  "descriptor_formats": {},
  "enabled": true,
  "environment": null,
  "id_length_limit": null,
  "label_key_case": null,
  "label_order": [],
  "label_value_case": null,
  "labels_as_tags": [
    "unset"
  ],
  "name": null,
  "namespace": null,
  "regex_replace_chars": null,
  "stage": null,
  "tags": {},
  "tenant": null
}

delimiter (string) optional

Delimiter to be used between ID elements.
Defaults to - (hyphen). Set to "" to use no delimiter at all.

Required: No

Default value: null

descriptor_formats (any) optional

Describe additional descriptors to be output in the descriptors output map.
Map of maps. Keys are names of descriptors. Values are maps of the form
\{<br/> format = string<br/> labels = list(string)<br/> \}
(Type is any so the map values can later be enhanced to provide additional options.)
format is a Terraform format string to be passed to the format() function.
labels is a list of labels, in order, to pass to format() function.
Label values will be normalized before being passed to format() so they will be
identical to how they appear in id.
Default is {} (descriptors output will be empty).

Required: No

Default value: { }

enabled (bool) optional

Set to false to prevent the module from creating any resources
Required: No

Default value: null

environment (string) optional

ID element. Usually used for region e.g. 'uw2', 'us-west-2', OR role 'prod', 'staging', 'dev', 'UAT'
Required: No

Default value: null

id_length_limit (number) optional

Limit id to this many characters (minimum 6).
Set to 0 for unlimited length.
Set to null for keep the existing setting, which defaults to 0.
Does not affect id_full.

Required: No

Default value: null

label_key_case (string) optional

Controls the letter case of the tags keys (label names) for tags generated by this module.
Does not affect keys of tags passed in via the tags input.
Possible values: lower, title, upper.
Default value: title.

Required: No

Default value: null

label_order (list(string)) optional

The order in which the labels (ID elements) appear in the id.
Defaults to ["namespace", "environment", "stage", "name", "attributes"].
You can omit any of the 6 labels ("tenant" is the 6th), but at least one must be present.

Required: No

Default value: null

label_value_case (string) optional

Controls the letter case of ID elements (labels) as included in id,
set as tag values, and output by this module individually.
Does not affect values of tags passed in via the tags input.
Possible values: lower, title, upper and none (no transformation).
Set this to title and set delimiter to "" to yield Pascal Case IDs.
Default value: lower.

Required: No

Default value: null

labels_as_tags (set(string)) optional

Set of labels (ID elements) to include as tags in the tags output.
Default is to include all labels.
Tags with empty values will not be included in the tags output.
Set to [] to suppress all generated tags.
Notes:
The value of the name tag, if included, will be the id, not the name.
Unlike other null-label inputs, the initial setting of labels_as_tags cannot be
changed in later chained modules. Attempts to change it will be silently ignored.

Required: No

Default value:

[
  "default"
]

name (string) optional

ID element. Usually the component or solution name, e.g. 'app' or 'jenkins'.
This is the only ID element not also included as a tag.
The "name" tag is set to the full id string. There is no tag with the value of the name input.

Required: No

Default value: null

namespace (string) optional

ID element. Usually an abbreviation of your organization name, e.g. 'eg' or 'cp', to help ensure generated IDs are globally unique
Required: No

Default value: null

regex_replace_chars (string) optional

Terraform regular expression (regex) string.
Characters matching the regex will be removed from the ID elements.
If not set, "/[^a-zA-Z0-9-]/" is used to remove all characters other than hyphens, letters and digits.

Required: No

Default value: null

stage (string) optional

ID element. Usually used to indicate role, e.g. 'prod', 'staging', 'source', 'build', 'test', 'deploy', 'release'
Required: No

Default value: null

tags (map(string)) optional

Additional tags (e.g. {'BusinessUnit': 'XYZ'}).
Neither the tag keys nor the tag values will be modified by this module.

Required: No

Default value: { }

tenant (string) optional

ID element (Rarely used, not included by default). A customer identifier, indicating who this instance of a resource is for
Required: No

Default value: null

Outputs

eks

Accounts with EKS and EKSs information

tgw_config

Transit Gateway config

transit_gateway_arn

Transit Gateway ARN

transit_gateway_id

Transit Gateway ID

transit_gateway_route_table_id

Transit Gateway route table ID

vpcs

Accounts with VPC and VPCs information

Dependencies

Requirements

• terraform, version: >= 1.0.0
• aws, version: >= 4.1

Modules

Name	Version	Source	Description
account_map	1.8.0	cloudposse/stack-config/yaml//modules/remote-state	n/a
eks	1.8.0	cloudposse/stack-config/yaml//modules/remote-state	n/a
iam_roles	latest	../../account-map/modules/iam-roles	n/a
tgw_hub	0.11.0	cloudposse/transit-gateway/aws	n/a
this	0.25.0	cloudposse/label/null	n/a
vpc	1.8.0	cloudposse/stack-config/yaml//modules/remote-state	n/a