Reference an existing virtual network in an Azure scale set template

Grein
08/23/2024

This article shows how to modify the basic scale set template to deploy into an existing virtual network instead of creating a new one.

Prerequisites

In a previous article we had created a basic scale set template. You will need that earlier template so that you can modify it to create a template that deploys a scale set into an existing virtual network.

Identify subnet

First, add a subnetId parameter. This string is passed into the scale set configuration, allowing the scale set to identify the pre-created subnet to deploy virtual machines into. This string must be of the form:

/subscriptions/<subscription-id>resourceGroups/<resource-group-name>/providers/Microsoft.Network/virtualNetworks/<virtual-network-name>/subnets/<subnet-name>

For instance, to deploy the scale set into an existing virtual network with name myvnet, subnet mysubnet, resource group myrg, and subscription 00000000-0000-0000-0000-000000000000, the subnetId would be:

/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/myrg/providers/Microsoft.Network/virtualNetworks/myvnet/subnets/mysubnet.

      },
      "adminPassword": {
        "type": "securestring"
+    },
+    "subnetId": {
+      "type": "string"
      }
    },

Delete extra virtual network resource

Next, delete the virtual network resource from the resources array, as you use an existing virtual network and don't need to deploy a new one.

    "variables": {},
    "resources": [
-    {
-      "type": "Microsoft.Network/virtualNetworks",
-      "name": "myVnet",
-      "location": "[resourceGroup().location]",
-      "apiVersion": "2018-11-01",
-      "properties": {
-        "addressSpace": {
-          "addressPrefixes": [
-            "10.0.0.0/16"
-          ]
-        },
-        "subnets": [
-          {
-            "name": "mySubnet",
-            "properties": {
-              "addressPrefix": "10.0.0.0/16"
-            }
-          }
-        ]
-      }
-    },

Remove dependency clause

The virtual network already exists before the template is deployed, so there is no need to specify a dependsOn clause from the scale set to the virtual network. Delete the following lines:

      {
        "type": "Microsoft.Compute/virtualMachineScaleSets",
        "name": "myScaleSet",
        "location": "[resourceGroup().location]",
        "apiVersion": "2019-03-01",
-      "dependsOn": [
-        "Microsoft.Network/virtualNetworks/myVnet"
-      ],
        "sku": {
          "name": "Standard_A1",
          "capacity": 2

Pass subnet parameter

Finally, pass in the subnetId parameter set by the user (instead of using resourceId to get the ID of a vnet in the same deployment, which is what the basic viable scale set template does).

                        "name": "myIpConfig",
                        "properties": {
                          "subnet": {
-                          "id": "[concat(resourceId('Microsoft.Network/virtualNetworks', 'myVnet'), '/subnets/mySubnet')]"
+                          "id": "[parameters('subnetId')]"
                          }
                        }
                      }