Allow creating VPC routes for already existing or shared TGW

Author: kamilhristov

examples/multi-account/main.tf

@@ -104,6 +104,9 @@ module "tgw_peer" {
       transit_gateway_default_route_table_association = false
       transit_gateway_default_route_table_propagation = false
 
+      vpc_route_table_ids  = module.vpc1.private_route_table_ids
+      tgw_destination_cidr = "0.0.0.0/0"
+
       tgw_routes = [
         {
           destination_cidr_block = "30.0.0.0/16"

main.tf

@@ -15,6 +15,7 @@ locals {
       for rtb_id in try(v.vpc_route_table_ids, []) : {
         rtb_id = rtb_id
         cidr   = v.tgw_destination_cidr
+        tgw_id = var.create_tgw ? aws_ec2_transit_gateway.this[0].id : v.tgw_id
       }
     ]
   ])
@@ -110,11 +111,14 @@ resource "aws_ec2_transit_gateway_route" "this" {
 }
 
 resource "aws_route" "this" {
-  for_each = { for x in local.vpc_route_table_destination_cidr : x.rtb_id => x.cidr }
+  for_each = { for x in local.vpc_route_table_destination_cidr : x.rtb_id => {
+    cidr   = x.cidr,
+    tgw_id = x.tgw_id
+  } }
 
   route_table_id         = each.key
-  destination_cidr_block = each.value
-  transit_gateway_id     = aws_ec2_transit_gateway.this[0].id
+  destination_cidr_block = each.value["cidr"]
+  transit_gateway_id     = each.value["tgw_id"]
 }
 
 resource "aws_ec2_transit_gateway_route_table_association" "this" {