Hello @Francis, Martin ,
Microsoft peering path lets you connect to Microsoft cloud services. The list of services includes Microsoft 365 services, such as Exchange Online, SharePoint Online, Skype for Business, and Microsoft Teams.
Azure virtual machines and cloud services that are deployed within a virtual network can be connected through the private peering.
You connect to virtual networks over the private peering path, and to other services over the Microsoft peering path.
The ExpressRoute gateway will advertise the Address Space(s) of the Azure VNet, you can't include/exclude at the subnet level. It is always the VNet Address Space that is advertised on ExpressRoute Private Peering.
Kindly let us know if the above helps or you need further assistance on this issue.
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