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Add IP address spaces to peered virtual networks

Virtual Network

Many organizations deploy a virtual networking architecture that follows the hub-spoke model. At some point, the hub virtual network might require additional IP address spaces. However, address ranges can't be added or deleted from a virtual network's address space once it's peered with another virtual network. To add or remove address ranges, delete the peering, add or remove the address ranges, then re-create the peering manually. The scripts described in this article can make that process easier.

Note

This article has not yet been updated to reflect Azure networking's support for peering resync. Azure virtual networks support adding and removing address space without the need to remove and restablish peerings; instead each remote peering needs a sync operation performed after the network space has changed. The sync can be performed using the Sync-AzVirtualNetworkPeering PowerShell command or from the Azure Portal.

Single subscription

A single subscription use case, both hub and all spoke virtual networks are in the same subscription.

Diagram that demonstrates a single subscription.

Multiple subscriptions

Another use case can be where the hub virtual network is in one subscription and all other spoke virtual networks are in different subscriptions. The subscriptions all share a single Azure Active Directory tenant.

Diagram that demonstrates multiple subscriptions.

Considerations

These considerations implement the pillars of the Azure Well-Architected Framework, which is a set of guiding tenets that can be used to improve the quality of a workload. For more information, see Microsoft Azure Well-Architected Framework.

  • Running the script will result in outage or disconnections between the hub and spoke virtual networks. Execute it during an approved maintenance window.
  • Run Get-Module -ListAvailable Az to find the installed version. The script requires the Azure PowerShell module version 1.0.0 or later. If you need to upgrade, see Install Azure PowerShell module.
  • If not already connected, run Connect-AzAccount to create a connection with Azure.
  • Consider assigning accounts, used for virtual network peering, to the Network Contributor role or a custom role containing the necessary actions found under virtual network peering permissions.
  • Assign accounts used to add IP address spaces, to the Network Contributor role or a custom role containing the necessary actions found under virtual network permissions.
  • The IP address space that you want to add to the hub virtual network must not overlap with any of the IP address spaces of the spoke virtual networks that you intend to peer with the hub virtual network.

Cost optimization

Cost optimization is about looking at ways to reduce unnecessary expenses and improve operational efficiencies. For more information, see Overview of the cost optimization pillar.

There is a nominal charge for ingress and egress traffic that utilizes a virtual network peering. There is no change to existing pricing when adding an additional IP address space to an Azure Virtual Network. For more information, see the pricing page.

Add the IP address range

The script automatically removes all Virtual Network peerings from the hub Virtual Network, adds an IP address range prefix to the hub Virtual Network based on input parameters, adds the Virtual Network peerings back to the hub Virtual Network, and reconnects the hub Virtual Network peerings to the existing spoke Virtual Network peerings. The script applies to single and multiple subscription hub-spoke topologies.

param (
    # Address Prefix range (CIDR Notation, e.g., 10.0.0.0/24 or 2607:f000:0000:00::/64)
    [Parameter(Mandatory = $true)]
    [String[]]
    $IPAddressRange,

    # Hub VNet Subscription Name
    [Parameter(Mandatory = $true)]
    [String]
    $HubVNetSubscriptionName,

    # Hub VNet Resource Group Name
    [Parameter(Mandatory = $true)]
    [String]
    $HubVNetRGName,

    # Hub VNet Name
    [Parameter(Mandatory = $true)]
    [String]
    $HubVNetName
)

#Set context to Hub VNet Subscription
Get-AzSubscription -SubscriptionName $HubVNetSubscriptionName | Set-AzContext
#end

#Get All Hub VNet Peerings and Hub VNet Object
$hubPeerings = Get-AzVirtualNetworkPeering -ResourceGroupName $HubVNetRGName -VirtualNetworkName $HubVNetName
$hubVNet = Get-AzVirtualNetwork -Name $HubVNetName -ResourceGroupName $HubVNetRGName
#end

#Remove All Hub VNet Peerings
foreach ($hubPeering in $hubPeerings) {
    Remove-AzVirtualNetworkPeering -VirtualNetworkName $HubVNetName -ResourceGroupName $HubVNetRGName -name $hubPeering.Name -Force
}
#end

#Add IP address range to the hub vnet
$hubVNet.AddressSpace.AddressPrefixes.Add($IPAddressRange)
#end

#Apply configuration stored in $hubVnet
Set-AzVirtualNetwork -VirtualNetwork $hubVNet
#end

foreach ($vNetPeering in $hubPeerings)
{
    # Get remote vnet name
    $vNetFullId = $vNetPeering.RemoteVirtualNetwork.Id
    $vNetName = $vNetFullId.Substring($vNetFullId.LastIndexOf('/') + 1)

    # Pull remote vNet object
    $vNetObj = Get-AzVirtualNetwork -Name $vNetName

    # Get the peering from the remote vnet object
    $peeringName = $vNetObj.VirtualNetworkPeerings.Where({$_.RemoteVirtualNetwork.Id -like "*$($hubVNet.Name)"}).Name
    $peering = Get-AzVirtualNetworkPeering -ResourceGroupName $vNetObj.ResourceGroupName -VirtualNetworkName $vNetName -Name $peeringName

    # Reset to initiated state
    Set-AzVirtualNetworkPeering -VirtualNetworkPeering $peering

    # Re-create peering on hub
    Add-AzVirtualNetworkPeering -Name $vNetPeering.Name -VirtualNetwork $HubVNet -RemoteVirtualNetworkId $vNetFullId -AllowGatewayTransit
    
    # Sync peering via remote vnet
    Sync-AzVirtualNetworkPeering -Name $peeringName -VirtualNetworkName $vNetName -ResourceGroupName $HubVNetRGName
}

Contributors

This article is maintained by Microsoft. It was originally written by the following contributors.

Principal author:

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Next steps

Read more about hub and spoke architecture:

Read more about virtual network architecture: