Microsoft released a new great feature, a few days ago, that allows us to add new address spaces into a VNet that has peered VNet:

How to re-size Azure virtual networks that are peered—now in preview | Azure Blog and Updates | Microsoft Azure

Before this feature, the only way to add a new address space to a subnet was to remove all VNet peering, add the new address space and redo peering. It was very difficult to do during the day because of the downtime that disabling the VNet peering caused.

Now, thanks to this feature, in 2 clicks you will be able to add a new address space, without interrupting your network.

Without this feature, here is the problem, if I try to add a new address space:

Address space

Error

You need to register the provider to have this new feature working. Don’t forget to re-register your Microsoft Network resource provider.

Go in your subscription, in Preview features and search for AllowUpdateAddressSpaceInPeeredVnets. Register it:

Preview features

After a few minutes, like every provider, the new one is registered:

Registered

Let’s now add a new address space range in our peered VNet:

 Add a new address space

If you now go to your peering, you will have a message that you need to sync the partner, to be able to route the traffic for the new subnet:

Peering

Select it and click on Sync:

Sync

After a few seconds, you can see that the status changed:

The status changed

You can now contact IP addresses in the new subnet from peers.

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