azure automactically changes subnet for vmss

Question

I have an image, I need to created a vm scaleset from it.

I have a vnet "vnet1" and different subnets ("LB-subnet","VMSS-subnet","Mangement-Subnet"). I created a load balancer in LB-subnet and it has frontend ip of 10.2.0.4. After that I created a VMSS(uniform) from my image and put it in vmss-subnet and attached the LB to it. But after the VMSS deployment, when i check its subnet, it is randomly selected. or may be the first subnet is selected. There is no option in vmss or its instances to change/add the nic just like we can in normal machines. is there any way to avoid it.

Answer 1

October 2023 and the bug is not solved. Azure Portal still assigns a random subnet to new VMSS.

I found a simpler way of changing subnet without creating a new NIC.

First, get the Resource ID of the subnet in the properties section of Azure Portal or in Azure Resource Explorer https://resources.azure.com/

After, replace the variable values of this Azure Powershell script and run it:

$ResourceGroupName = "<ResourceGroupName>"
$ScaleSetName = "<ScaleSetName>"
$NewSubnetResourceId = "<NewSubnetResourceId>"

$vmss = Get-AzVmss -ResourceGroupName $ResourceGroupName -VMScaleSetName $ScaleSetName
$vmss.VirtualMachineProfile.NetworkProfile.NetworkInterfaceConfigurations[0].IpConfigurations[0].Subnet.Id = $NewSubnetResourceId
Update-AzVmss -ResourceGroupName $ResourceGroupName -VMScaleSetName $ScaleSetName -VirtualMachineScaleSet $vmss

Answer 2

This bug is still present in the Azure portal when creating a VMSS into an existing VNet with multiple subnets: Whatever subnet the VMSS NIC defaulted to when selecting an existing VNet will be the one it deploys to, even though you changed it in the NIC settings.

Answer 3

Hello @Muhammed Noor ,
Thank you for reaching out to Microsoft QnA.
I have done a repro of the same, and I see that every time the subnet of the VMSS gets changed to the subnet of the load balancer. This makes me think that it is the default behavior when we attach the vmss to a load balancer.
You can however add a secondary nic to your VMSS using this Powershell command and then update the VMSS. Once you update the VMSS, the secondary nic will be reflected at each of the VMSS instance level.

Hope this helps. If you have any queries, feel free to reach back.

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