Note: This question is part of a series of questions that present the same scenario.

In Azure, when using a Standard SKU Load Balancer, all virtual machines (VMs) that are members of the backend pool must meet specific configuration requirements defined by Microsoft.

According to the Azure Load Balancer documentation (Microsoft Learn: “Standard Load Balancer overview” and “Backend pool management”), the following rules apply:

1️⃣ All VMs in the backend pool must be in the same virtual network (VNet) as the load balancer.

2️⃣ All network interfaces (NICs) attached to backend pool members must use Standard SKU public or private IP configurations—you cannot mix Basic and Standard SKUs.

3️⃣ You cannot associate public IPs directly with VM NICs for internal load balancers. Internal load balancers use private IPs to route traffic within a VNet.

In the proposed solution, you are associating a Standard SKU public IP address to VM1’s NIC. However, since LB1 is an internal load balancer, it operates only within a private network scope—public IP associations are not applicable. Also, stopping VM2 does not affect its ability to be added to the backend pool; what matters is its network configuration.

The correct solution would be to ensure that both VM1 and VM2 have network interfaces connected to the same VNet (VNET1) and configured with Standard SKU IP configurations (not public).