Cannot add VM to internal load balancer pool when VM exists outside of load balancer VNET

Question

When using the Microsoft internal load balancer (ILB), I would like to create a pool for VMs that reside in a VNET that is a different than the VNET where the ILB is located. The UI would seem to support this as I can select any VNET in my environment when creating the pool. Yet, when I create this pool, I receive the following error that would imply this is not allowed.

NetworkInterfaceAndInternalLoadBalancerMustUseSameVnet

{"code":"BadRequest","message":"{\r\n \"error\": {\r\n \"code\": \"NetworkInterfaceAndInternalLoadBalancerMustUseSameVnet\",\r\n \"message\": \"Network interface /subscriptions/2f46d973-XXXX-XXXX-80a7-7222a103acb4/resourceGroups/ihde_operations/providers/Microsoft.Network/networkInterfaces/op-vm-ftp1463 uses internal load balancer /subscriptions/2f46d973-cea1-XXXX-XXXX-7222a103acb4/resourceGroups/ihde_dev/providers/Microsoft.Network/loadBalancers/dev-lb-CSL-Internal but does not use the same VNET (/subscriptions/2f46d973-cea1-4856-80a7-7222a103acb4/resourceGroups/IHDE_DEV/providers/Microsoft.Network/virtualNetworks/VNET_BACKEND) as the load balancer.\",\r\n \"details\": []\r\n }\r\n}"}]}

As a side note, the public version of the load balancer does support this this scenario without any issues.

Answer 1

Per this doc.

An internal Load Balancer differs from a public Load Balancer. Azure infrastructure restricts access to the load-balanced frontend IP addresses of a virtual network.

For an internal Load Balancer, It enables load balancing of VMs in a virtual network to a set of VMs that reside within the same virtual network. So you could not create a pool for VMs that reside in a VNET that is a different than the VNET where the ILB is located.