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Routing

Overview

Routes define the paths network traffic takes from VM instances to destinations. GCP automatically creates routes for subnets and provides options for custom routes to control traffic flow.

Table of Contents

  1. Routing Fundamentals
  2. Route Types
  3. Custom Routes
  4. Route Priority
  5. Best Practices

Routing Fundamentals

How Routing Works

┌────────────────────────────────────────────────────────┐
│              Route Selection Process                   │
└────────────────────────────────────────────────────────┘

Packet Destination: 10.0.2.5
    │
    ├─→ Check Routes (by specificity, then priority)
    │   ├─ Route 1: 10.0.2.0/24 (priority 1000) ✓ Most specific
    │   ├─ Route 2: 10.0.0.0/16 (priority 1000)
    │   └─ Route 3: 0.0.0.0/0   (priority 1000)
    │
    └─→ Use Route 1 (most specific match)

System-Generated Routes

# View all routes
gcloud compute routes list

# Routes automatically created:
# 1. Subnet routes (one per subnet)
# 2. Default internet gateway route

Route Types

1. Subnet Routes

# Automatically created for each subnet
route:
  name: "default-route-subnet"
  destination: "10.0.1.0/24"
  next_hop: "VPC network"
  priority: 1000

# Cannot be deleted
# Deleted only when subnet is deleted

2. Default Route

# Default internet gateway route
route:
  name: "default-route-internet"
  destination: "0.0.0.0/0"
  next_hop: "default-internet-gateway"
  priority: 1000

# Can be deleted if needed
# Recreate with: gcloud compute routes create

3. Peering Routes

# Automatically created for VPC peering
route:
  name: "peering-route"
  destination: "10.1.0.0/16"  # Peer VPC range
  next_hop: "peering-connection"
  priority: 1000

Custom Routes

Static Routes

# Create static route
gcloud compute routes create route-to-onprem \
    --network=my-vpc \
    --destination-range=192.168.0.0/16 \
    --next-hop-gateway=vpn-gateway \
    --priority=1000

# Route to specific instance
gcloud compute routes create route-to-appliance \
    --network=my-vpc \
    --destination-range=172.16.0.0/12 \
    --next-hop-instance=firewall-vm \
    --next-hop-instance-zone=us-central1-a \
    --priority=1000

# Route to IP address
gcloud compute routes create route-to-ip \
    --network=my-vpc \
    --destination-range=10.2.0.0/16 \
    --next-hop-address=10.0.1.10 \
    --priority=1000

# Delete route
gcloud compute routes delete route-to-onprem

Terraform

# Static route to on-premises
resource "google_compute_route" "to_onprem" {
  name             = "route-to-onprem"
  network          = google_compute_network.vpc.name
  dest_range       = "192.168.0.0/16"
  next_hop_gateway = "projects/${var.project_id}/global/gateways/vpn-gateway"
  priority         = 1000
  tags             = ["vpn-route"]
}

# Route through network appliance
resource "google_compute_route" "through_appliance" {
  name                   = "route-through-appliance"
  network                = google_compute_network.vpc.name
  dest_range             = "172.16.0.0/12"
  next_hop_instance      = google_compute_instance.firewall.id
  next_hop_instance_zone = "us-central1-a"
  priority               = 1000
}

# Route to internal IP
resource "google_compute_route" "to_internal_ip" {
  name         = "route-to-internal-ip"
  network      = google_compute_network.vpc.name
  dest_range   = "10.2.0.0/16"
  next_hop_ip  = "10.0.1.10"
  priority     = 1000
}

# Default route override
resource "google_compute_route" "custom_default" {
  name             = "custom-default-route"
  network          = google_compute_network.vpc.name
  dest_range       = "0.0.0.0/0"
  next_hop_gateway = "default-internet-gateway"
  priority         = 900  # Higher priority than default
  tags             = ["internet-access"]
}

Route Priority

Priority Rules

Priority Range: 0 - 65535
├─ 0: Highest priority
├─ 1000: Default for system routes
└─ 65535: Lowest priority

Selection:
1. Most specific destination range
2. If equal specificity, lowest priority number wins

Examples

# High priority route (overrides default)
gcloud compute routes create priority-route \
    --network=my-vpc \
    --destination-range=0.0.0.0/0 \
    --next-hop-gateway=custom-gateway \
    --priority=100

# Normal priority
gcloud compute routes create normal-route \
    --network=my-vpc \
    --destination-range=10.2.0.0/16 \
    --next-hop-address=10.0.1.10 \
    --priority=1000

# Low priority backup route
gcloud compute routes create backup-route \
    --network=my-vpc \
    --destination-range=10.2.0.0/16 \
    --next-hop-address=10.0.1.20 \
    --priority=2000

Best Practices

1. Use Specific Routes

✓ More specific routes take precedence 

# Good: Specific routes
resource "google_compute_route" "specific" {
  name       = "route-to-subnet"
  network    = google_compute_network.vpc.name
  dest_range = "10.0.1.0/24"  # Specific
  next_hop_ip = "10.0.0.1"
  priority   = 1000
}

# Avoid: Overly broad routes

2. Document Routes

resource "google_compute_route" "to_onprem" {
  name       = "route-to-onprem"
  network    = google_compute_network.vpc.name
  dest_range = "192.168.0.0/16"
  next_hop_gateway = google_compute_vpn_gateway.vpn.id
  priority   = 1000

  description = "Route to on-premises network via VPN. Required for hybrid connectivity."
}

3. Use Tags for Selective Routing

# Route applies only to tagged instances
resource "google_compute_route" "selective" {
  name       = "route-for-specific-vms"
  network    = google_compute_network.vpc.name
  dest_range = "10.2.0.0/16"
  next_hop_ip = "10.0.1.10"
  priority   = 1000
  tags       = ["custom-routing"]
}

Summary

Routing in GCP: - Automatic subnet routes - Custom static routes - Priority-based selection - Tag-based application

Quick Reference

# List routes
gcloud compute routes list --filter="network:my-vpc"

# Create route
gcloud compute routes create ROUTE \
    --network=VPC --dest-range=CIDR \
    --next-hop-address=IP --priority=1000

# Delete route
gcloud compute routes delete ROUTE

Next Steps

Last Updated: March 2026