main.tf

provider "aws" {
  region = var.aws_region
}

variable "aws_region" {
  description = "The AWS region to deploy resources in."
  default     = "us-west-2"
}

variable "vpc_a_id" {
  description = "ID for existing VPC A"
  type        = string
}

variable "vpc_b_id" {
  description = "ID for existing VPC B"
  type        = string
}

variable "vpc_a_route_table_ids" {
  description = "List of route table IDs for VPC A"
  type        = list(string)
  default     = []
}

variable "vpc_b_route_table_ids" {
  description = "List of route table IDs for VPC B"
  type        = list(string)
  default     = []
}

resource "aws_vpc_peering_connection" "peer_a_to_b" {
  vpc_id      = var.vpc_a_id
  peer_vpc_id = var.vpc_b_id
  auto_accept = true

  tags = {
    Name = "vpc_a_to_vpc_b"
  }
}

resource "aws_route" "route_a_to_b" {
  count                     = length(var.vpc_a_route_table_ids)
  route_table_id            = var.vpc_a_route_table_ids[count.index]
  destination_cidr_block    = var.vpc_b_cidr  # We would need the CIDR for this to work, or can be made variable.
  vpc_peering_connection_id = aws_vpc_peering_connection.peer_a_to_b.id
}

resource "aws_route" "route_b_to_a" {
  count                     = length(var.vpc_b_route_table_ids)
  route_table_id            = var.vpc_b_route_table_ids[count.index]
  destination_cidr_block    = var.vpc_a_cidr  # Same comment as above.
  vpc_peering_connection_id = aws_vpc_peering_connection.peer_a_to_b.id
}

# Optional: If you don't know the CIDRs of the VPCs by heart, 
# you can use data blocks to fetch them based on VPC ID
data "aws_vpc" "vpc_a_data" {
  id = var.vpc_a_id
}

data "aws_vpc" "vpc_b_data" {
  id = var.vpc_b_id
}

terraform.tfvars

aws_region             = "us-east-1"
vpc_a_id               = "vpc-0123456789abcdef0"
vpc_b_id               = "vpc-abcdef0123456789"
vpc_a_route_table_ids  = ["rtb-0123456789abcdef0", "rtb-0123456789abcdef1"]
vpc_b_route_table_ids  = ["rtb-abcdef0123456789", "rtb-abcdef012345678a"]