VNet Peering Constraints

Constraints and what is not possible with VNet Peering:

* Peering virtual networks in different regions is also referred to as global peering. When creating a global peering, the peered virtual networks can exist in any Azure public cloud region, but not in Azure national clouds. You can only peer virtual networks in the same region in national clouds.

* Resources in one virtual network cannot communicate with the front-end IP address of an Azure internal load balancer in a globally peered virtual network. The load balancer and the resources that communicate with it must be in a virtual network in the same region.

* You cannot use remote gateways or allow gateway transit in globally peered virtual networks.

* The virtual networks you peer cannot have overlapping IP address spaces.

* You cannot add address ranges to, or delete address ranges from a virtual network's address space once a virtual network is peered with another virtual network.

* You cannot peer two virtual networks created through the classic deployment model.

* Peerings are not transitive. If you create peerings between: VirtualNetwork1 & VirtualNetwork2
and VirtualNetwork2 & VirtualNetwork3, there is no peering between VirtualNetwork1 and VirtualNetwork3 through VirtualNetwork2.

* You cannot resolve names in peered virtual networks using default Azure name resolution. To resolve names in other virtual networks, you must use Azure DNS for private domains or a custom DNS server.

* There is a nominal charge for ingress and egress traffic that utilizes a virtual network peering.

VNet peering pricing is calculated differently than VNet-to-VNet VPN Gateway pricing.

Source: Official Azure documentation