Adding an unattached new NIC to a subnet "resets" my vnet parameters

Question

Adding an unattached new NIC to a subnet "resets" my vnet parameters

Dufour, Francois 56

Hi,

I have a really really weird behaviour on an Azure VNET. I have a VNET with a /21 address space, a few /27 subnets, custom dns specified and a peering. I then created a new NIC, plugged to one of the subnets and not attached to any VM (plan was to add a secondary NIC to a VM). The result of that is... (get ready), NIC properly get an IP from the subnet, DNS Settings at vnet level are revert to Azure default andddddd my /21 address space is modified and replaced by several /27 address spaces corresponding to my list of subnets... When discovering the change the first time, checking the activity log and discovering the change was initiated by... me I was wondering I got some head/concentration issues then managed to find what was causing all this....

I managed to reproduce the behaviour in this environment and not in another one. I have absolutly no idea what could be the reason for such a behaviour, I checked the vnet properties and policies but anyway what would be the point of this.

Have anyone ever encounter this ?

ChaitanyaNaykodi-MSFT 27,476 Reputation points Microsoft Employee Moderator

2023-07-04T02:42:42.5+00:00

@Dufour, Francois

Thank you for reaching out. Can you please confirm if the NIC was created via Portal?
As documented here limited options are available when you are creating NIC via portal and more options are available via PowerShell and CLI.

Can you please share a screenshot/command for the options selected while creating the NIC. Thank you!
Dufour, Francois 56 Reputation points

2023-07-04T10:35:17.3+00:00

Nothing exotic here...
JimmySalian-2011 42,491 Reputation points

2023-07-04T11:10:43.15+00:00

Can you provide your Subnet range allocation and VNET Address space allocation, I am suspecting if addresses are not available azure will assign the dynamic available address. For example, if the subnet's address range is 10.0.0.0/16, and addresses 10.0.0.4-10.0.0.14 are already assigned (.0-.3 are reserved), Azure assigns 10.0.0.15 to the resource. Dynamic is the default allocation method. Once assigned, dynamic IP addresses are only released if a network interface is deleted, assigned to a different subnet within the same virtual network, or the allocation method is changed to static, and a different IP address is specified. Do you have enough addresses available?

Can you check if there are any policies that is overriding the settings?
Dufour, Francois 56 Reputation points

2023-07-04T11:26:56.7266667+00:00

My subnets are large enough I still have about 25 available IPs in each. The capture here shows the change traced in the Activity Log. As I explained, you can see that my DNS settings are erased and that my address space is replaced by the list of my subnets.
Joe Carlyle 661 Reputation points MVP

2023-07-05T07:49:08.1466667+00:00

This is bizarre, so very interesting!

Based on the address space change, I presume there are no other resources in that VNET?

Does the same change occur with a different subnet in that VNET?

Does the same change occur if you choose static IP assignment at deployment?

Accepted answer

0 additional answers

Your answer

ChaitanyaNaykodi-MSFT 27,476 Reputation points Microsoft Employee Moderator

2023-07-04T02:42:42.5+00:00

@Dufour, Francois

Thank you for reaching out. Can you please confirm if the NIC was created via Portal?
As documented here limited options are available when you are creating NIC via portal and more options are available via PowerShell and CLI.

Can you please share a screenshot/command for the options selected while creating the NIC. Thank you!
Dufour, Francois 56 Reputation points

2023-07-04T10:35:17.3+00:00

Nothing exotic here...
JimmySalian-2011 42,491 Reputation points

2023-07-04T11:10:43.15+00:00

Can you provide your Subnet range allocation and VNET Address space allocation, I am suspecting if addresses are not available azure will assign the dynamic available address. For example, if the subnet's address range is 10.0.0.0/16, and addresses 10.0.0.4-10.0.0.14 are already assigned (.0-.3 are reserved), Azure assigns 10.0.0.15 to the resource. Dynamic is the default allocation method. Once assigned, dynamic IP addresses are only released if a network interface is deleted, assigned to a different subnet within the same virtual network, or the allocation method is changed to static, and a different IP address is specified. Do you have enough addresses available?

Can you check if there are any policies that is overriding the settings?
Dufour, Francois 56 Reputation points

2023-07-04T11:26:56.7266667+00:00

My subnets are large enough I still have about 25 available IPs in each. The capture here shows the change traced in the Activity Log. As I explained, you can see that my DNS settings are erased and that my address space is replaced by the list of my subnets.
Joe Carlyle 661 Reputation points MVP

2023-07-05T07:49:08.1466667+00:00

This is bizarre, so very interesting!

Based on the address space change, I presume there are no other resources in that VNET?

Does the same change occur with a different subnet in that VNET?

Does the same change occur if you choose static IP assignment at deployment?

Answer 1

ChaitanyaNaykodi-MSFT 27,476 Microsoft Employee Moderator

@Dufour, Francois

Thank you for getting back, based on the discussion above I think a support ticket will be required to get to the bottom of the issue as a support engineer can take a look at the backend logs and determine the exact cause.

If you have a support plan you may file a support ticket, else could you please send an email to azcommunity@microsoft.com with the below details. I will help you with a one-time free support in this case.

Subject : Attn Chaitanya

Thread URL: Link to this thread.

Subscription ID

Please let me know once you have done the same.

Dufour, Francois 56 Reputation points

2023-07-07T09:59:15.9166667+00:00

thank you very much, just sent a mail with the informations
Dufour, Francois 56 Reputation points

2023-07-27T12:31:53.8733333+00:00

Hello, mail has been sent on the 7th of July, forgot to ping you here...
ChaitanyaNaykodi-MSFT 27,476 Reputation points Microsoft Employee Moderator

2023-08-01T03:52:16.5333333+00:00

@Dufour, Francois

Thank you for getting back. I did reply to your email on 7th of July. I will bump it again in your inbox. Please let me know if you are unable to see it.

Share via

Adding an unattached new NIC to a subnet "resets" my vnet parameters

0 additional answers

Your answer