Add Additional Subnets to Virtual Network Without Affecting its Existing Config

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Add Additional Subnets to Virtual Network Without Affecting its Existing Config

Taranjeet Malik 566

Hi

I'm in a situation where I need to add additional subnets and NSG config to those subnets for an existing Virtual Network using Bicep. The existing Virtual Network and its subnet configuration was deployed in the past using mixed methods (ClickOps and some JSON ARM templates). My question is, can I reference the existing Virtual Network using the "existing" clause in Bicep and try to add new subnets to it without affecting / overwriting the existing subnets?

If not, what are my options in this case?

Thanks

Taranjeet Singh

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Answer 1

KapilAnanth-MSFT 49,521 Microsoft Employee Moderator

@Taranjeet Malik ,

Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.

I understand that you would like to deploy a subnet to an existing VNET using Bicep without disrupting the existing subnet configurations.

Yes, this is doable.

You can use the below bicep configuration example to achieve this.

var subnetName = 'newSubnet'
var vnetName = 'vnet'

resource myvnet 'Microsoft.Network/virtualNetworks@2023-04-01' existing = {
  name: vnetName
}

resource subnet 'Microsoft.Network/virtualNetworks/subnets@2023-04-01' = {

  name: subnetName
  parent: myvnet
  properties:{
    addressPrefix:'10.0.20.0/24' //Address prefix should **not** be overlapping with existing subnets
  }

}

Thanks,

Kapil

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Taranjeet Malik 566 Reputation points

2024-04-29T07:50:20.2966667+00:00

Hi @KapilAnanth-MSFT

Thanks for the response. I see that you mentioned this as a comment "//Address prefix should be overlapping with existing subnets" - wouldn't that overwrite the existing subnet(s)?

I want to add additional subnets and do not intend to overwrite / modify any of the existing VNet config, such as existing DNS servers, subnets, NSGs, and Route Tables to name a few.

I referred to this article, and it states even the "incremental" mode would need us to specify the values for all the existing resources, otherwise they'll be set to default.

https://learn.microsoft.com/en-us/azure/azure-resource-manager/templates/deployment-modes

Am I reading something incorrectly?
KapilAnanth-MSFT 49,521 Reputation points Microsoft Employee Moderator

2024-04-29T07:58:17.8666667+00:00
@Taranjeet Malik ,

My sincere apologies.

The comment was supposed to be , "Address prefix should not be overlapping with existing subnets".

I will update the code now in the answer.

In your case, where you want to add new resources without updating existing resources, the bicep code I provided is correct.

It will not modify the existing subnets' configurations.

It will only create new subnet as long as the Address prefix does not overlap with other subnets in the VNET.

Kindly let us know if this helps or you need further assistance on this issue.

Thanks,

Kapil

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Taranjeet Malik 566 Reputation points

2024-05-01T23:51:55.9933333+00:00

Hi @KapilAnanth-MSFT

Thanks for sharing the piece of code. I tested that and it works. Appreciate the inputs there. Just another clarification then, is this applicable to any resource deployed to Azure or there are any exceptions / things to be aware of when modifying resources using Bicep that were previously deployed through non-Bicep means (ClickOps, Terraform, CLI etc.)?

Also, not sure what's suggested in the article then--> https://learn.microsoft.com/en-us/azure/azure-resource-manager/templates/deployment-modes

Can you please shed some light on that pls.?
KapilAnanth-MSFT 49,521 Reputation points Microsoft Employee Moderator

2024-05-02T04:01:15.8+00:00
@Taranjeet Malik ,

I am not aware of any specific examples as such.

It is generally suggested to be cautious with child resources.

In this specific example, if you were to deploy the VNET altogether, without specifying the existing subnets, it will overwrite the VNET configuration to be without any subnets

i.e., in case you redeploy the VNET - you are expected to properly define the new subnets and also existing subnets within it, else the redeployment will only create the VNET and the new subnets.

However, with the use of "existing" keyword , the VNET retains the existing configuration of subnets, and from there you are free to deploy new subnets (child resources) without worrying about the existing subnets.

i.e., in this case, there is no redeployment of the VNET at all, you are simply using this VNET as a reference for your subnet (child resource)

Hope this helps.

Thanks,

Kapil

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Add Additional Subnets to Virtual Network Without Affecting its Existing Config

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