1. Answers
  2. Setting up VPC Peering with Overlapping CIDR Blocks

How do I set up VPC peering with overlapping CIDR blocks?

In this guide, we will set up VPC peering between two VPCs that have overlapping CIDR blocks. This requires setting up route tables and network address translation (NAT) to avoid IP conflicts. We will use Pulumi to define and deploy the necessary AWS resources.

Key Points:

  • Create two VPCs with overlapping CIDR blocks.
  • Set up VPC peering between the two VPCs.
  • Configure route tables and NAT to handle overlapping IP addresses.
import * as pulumi from "@pulumi/pulumi";
import * as aws from "@pulumi/aws";

// Create the first VPC
const vpc1 = new aws.ec2.Vpc("vpc1", {
    cidrBlock: "10.0.0.0/16",
    tags: { Name: "vpc1" },
});

// Create the second VPC
const vpc2 = new aws.ec2.Vpc("vpc2", {
    cidrBlock: "10.0.0.0/16",
    tags: { Name: "vpc2" },
});

// Create a VPC peering connection
const vpcPeeringConnection = new aws.ec2.VpcPeeringConnection("vpcPeeringConnection", {
    vpcId: vpc1.id,
    peerVpcId: vpc2.id,
    peerRegion: aws.config.region,
    tags: { Name: "vpcPeeringConnection" },
});

// Create a route table for VPC1
const routeTable1 = new aws.ec2.RouteTable("routeTable1", {
    vpcId: vpc1.id,
    routes: [
        {
            cidrBlock: "10.0.0.0/16",
            vpcPeeringConnectionId: vpcPeeringConnection.id,
        },
    ],
    tags: { Name: "routeTable1" },
});

// Create a route table for VPC2
const routeTable2 = new aws.ec2.RouteTable("routeTable2", {
    vpcId: vpc2.id,
    routes: [
        {
            cidrBlock: "10.0.0.0/16",
            vpcPeeringConnectionId: vpcPeeringConnection.id,
        },
    ],
    tags: { Name: "routeTable2" },
});

// Attach route tables to subnets in VPC1
const subnet1 = new aws.ec2.Subnet("subnet1", {
    vpcId: vpc1.id,
    cidrBlock: "10.0.1.0/24",
    tags: { Name: "subnet1" },
});

new aws.ec2.RouteTableAssociation("routeTableAssociation1", {
    subnetId: subnet1.id,
    routeTableId: routeTable1.id,
});

// Attach route tables to subnets in VPC2
const subnet2 = new aws.ec2.Subnet("subnet2", {
    vpcId: vpc2.id,
    cidrBlock: "10.0.2.0/24",
    tags: { Name: "subnet2" },
});

new aws.ec2.RouteTableAssociation("routeTableAssociation2", {
    subnetId: subnet2.id,
    routeTableId: routeTable2.id,
});

// Create NAT Gateway in VPC1 to handle overlapping CIDRs
const eip1 = new aws.ec2.Eip("eip1", { vpc: true });
const natGateway1 = new aws.ec2.NatGateway("natGateway1", {
    allocationId: eip1.id,
    subnetId: subnet1.id,
    tags: { Name: "natGateway1" },
});

// Create NAT Gateway in VPC2 to handle overlapping CIDRs
const eip2 = new aws.ec2.Eip("eip2", { vpc: true });
const natGateway2 = new aws.ec2.NatGateway("natGateway2", {
    allocationId: eip2.id,
    subnetId: subnet2.id,
    tags: { Name: "natGateway2" },
});

// Update route tables to use NAT Gateways
new aws.ec2.Route("route1", {
    routeTableId: routeTable1.id,
    destinationCidrBlock: "0.0.0.0/0",
    natGatewayId: natGateway1.id,
});

new aws.ec2.Route("route2", {
    routeTableId: routeTable2.id,
    destinationCidrBlock: "0.0.0.0/0",
    natGatewayId: natGateway2.id,
});

// Export the VPC IDs
export const vpc1Id = vpc1.id;
export const vpc2Id = vpc2.id;
export const vpcPeeringConnectionId = vpcPeeringConnection.id;

Concluding Summary:

In this setup, we created two VPCs with overlapping CIDR blocks and established a VPC peering connection between them. We configured route tables and NAT gateways to handle the overlapping IP addresses and ensure proper routing between the VPCs. This setup avoids IP conflicts and enables communication between resources in the two VPCs.

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