Backend pool management

The backend pool is a critical component of the load balancer. The backend pool defines the group of resources that serve traffic for a given load-balancing rule.

There are two ways of configuring a backend pool:

1. Network Interface Card (NIC)

2. IP address

To preallocate a backend pool with an IP address range that contains virtual machines and Virtual Machine Scale Sets, configure the pool by IP address and virtual network ID. IP-based backend pools require a Standard Load Balancer.

This article focuses on configuration of IP-based backend pools.

Configure backend pool by IP address and virtual network

In scenarios with prepopulated backend pools, use IP addresses and a virtual network.

Before you begin, ensure that you have:

• An existing Standard Load Balancer and virtual network.
• Backend resources in the same virtual network as the load balancer.
• Permission to create or update load balancer, network interface, and virtual machine resources.
• An authenticated Azure PowerShell or Azure CLI session for the tool you use.

You configure backend pool management on the backend pool object as highlighted in the following examples.

PowerShell

Create a new backend pool:

$be = @{
    ResourceGroupName = 'myResourceGroup'
    LoadBalancerName = 'myLoadBalancer'
    Name = 'myBackendPool'
}
$backendPool = New-AzLoadBalancerBackendAddressPool @be

Update backend pool with a new IP from existing virtual network:

$vnet = @{
    Name = 'myVnet'
    ResourceGroupName = 'myResourceGroup'
}
$virtualNetwork = Get-AzVirtualNetwork @vnet

$add1 = @{
    IpAddress = '10.0.0.5'
    Name = 'TestVNetRef'
    VirtualNetworkId = $virtualNetwork.Id
}
$ip1 = New-AzLoadBalancerBackendAddressConfig @add1

$backendPool.LoadBalancerBackendAddresses.Add($ip1) 

Set-AzLoadBalancerBackendAddressPool -InputObject $backendPool

Retrieve the backend pool information for the load balancer to confirm that the backend addresses are added to the backend pool:

$pool = @{
    ResourceGroupName = 'myResourceGroup'
    LoadBalancerName = 'myLoadBalancer'
    Name = 'myBackendPool'
}
Get-AzLoadBalancerBackendAddressPool @pool

Create a network interface and add it to the backend pool. Set the IP address to one of the backend addresses:

$net = @{
    Name = 'myNic'
    ResourceGroupName = 'myResourceGroup'
    Location = 'chinaeast2'
    PrivateIpAddress = '10.0.0.5'
    Subnet = $virtualNetwork.Subnets[0]
}
$nic = New-AzNetworkInterface @net

Create a VM and attach the NIC with an IP address in the backend pool:

# Create a username and password for the virtual machine
$cred = Get-Credential

# Create a virtual machine configuration
$net = @{
    Name = 'myNic'
    ResourceGroupName = 'myResourceGroup'
}
$nic = Get-AzNetworkInterface @net

$vmc = @{
    VMName = 'myVM1'
    VMSize = 'Standard_DS1_v2'
}

$vmos = @{
    ComputerName = 'myVM1'
    Credential = $cred
}

$vmi = @{
    PublisherName = 'MicrosoftWindowsServer'
    Offer = 'WindowsServer'
    Skus = '2019-Datacenter'
    Version = 'latest'
}
$vmConfig = 
New-AzVMConfig @vmc | Set-AzVMOperatingSystem -Windows @vmos | Set-AzVMSourceImage @vmi | Add-AzVMNetworkInterface -Id $nic.Id

# Create a virtual machine using the configuration
$vm = @{
    ResourceGroupName = 'myResourceGroup'
    Zone = '1'
    Location = 'chinanorth3'
    VM = $vmConfig

}
$vm1 = New-AzVM @vm

CLI

By using Azure CLI, you can populate the IP-based backend pool through command-line parameters or a JSON configuration file.

Create an IP-based backend pool with Azure CLI

Create and populate the backend pool via the command-line parameters:

az network lb address-pool create \
--resource-group myResourceGroup \
--lb-name myLB \
--name myBackendPool \
--vnet {VNET resource ID} \
--backend-address name=addr1 ip-address=10.0.0.4 \
--backend-address name=addr2 ip-address=10.0.0.5

Create and populate the Backend Pool via JSON configuration file:

az network lb address-pool create \
--resource-group myResourceGroup \
--lb-name myLB \
--name myBackendPool \
--vnet {VNET resource ID} \
--backend-address-config-file @config_file.json

JSON configuration file:

        [
          {
            "name": "address1",
            "virtualNetwork": "/subscriptions/{subscriptionId}/resourceGroups/{resource-group-name}/providers/Microsoft.Network/virtualNetworks/{vnet-name}",
            "ipAddress": "10.0.0.4"
          },
          {
            "name": "address2",
            "virtualNetwork": "/subscriptions/{subscriptionId}/resourceGroups/{resource-group-name}/providers/Microsoft.Network/virtualNetworks/{vnet-name}",
            "ipAddress": "10.0.0.5"
          }
        ]

Verify backend addresses with Azure CLI

Retrieve the backend pool information for the load balancer to confirm that the backend addresses are added to the backend pool:

az network lb address-pool show \
--resource-group myResourceGroup \
--lb-name MyLb \
--name MyBackendPool

Create a network interface with Azure CLI

Create a network interface and add it to the backend pool. Set the IP address to one of the backend addresses:

az network nic create \
--resource-group myResourceGroup \
--name myNic \
--vnet-name myVnet \
--subnet mySubnet \
--network-security-group myNetworkSecurityGroup \
--lb-name myLB \
--private-ip-address 10.0.0.4

Create a virtual machine with Azure CLI

Create a VM and attach the NIC with an IP address in the backend pool:

az vm create \
--resource-group myResourceGroup \
--name myVM \
--nics myNic \
--image Ubuntu2204 \
--admin-username azureuser \
--generate-ssh-keys

Limitations for IP-based backend pools

• You can only use IP-based backend pools with Standard Load Balancers.
• The backend resources must be in the same virtual network as the load balancer.
• Backend instances in an IP-based backend pool must be virtual machines or virtual machine scale sets. You can't attach other PaaS services.
• A load balancer with an IP-based backend pool can't function as a Private Link service.
• You can't place Private endpoint resources in an IP-based backend pool.
• IP-based backend pools don't support ACI containers.
• Load balancers or services such as Application Gateway can't be placed in the backend pool of the load balancer.
• Inbound NAT Rules can't be specified by IP address.
• You can configure both IP-based and NIC-based backend pools for the same load balancer. You can't create a single backend pool that mixes backend addresses targeted by NIC and IP address within the same pool.
• A virtual machine in the same virtual network as an internal load balancer can't access the frontend of the ILB and its backend VMs simultaneously.

Next steps

In this article, you learned about Azure Load Balancer backend pool management and how to configure a backend pool by IP address and virtual network.

Learn more about Azure Load Balancer.

Review the REST API for IP based backend pool management.