The vCenter Server must set the distributed port group Forged Transmits policy to "Reject".
If the virtual machine operating system changes the Media Access Control (MAC) address, the operating system can send frames with an impersonated source MAC address at any time. This allows an operating system to stage malicious attacks on the devices in a network by impersonating a network adaptor authorized by the receiving network.
When the "Forged Transmits" option is set to "Accept", ESXi does not compare source and effective MAC addresses.
To protect against MAC impersonation, set the "Forged Transmits" option to "Reject". The host compares the source MAC address being transmitted by the guest operating system with the effective MAC address for its virtual machine adapter to determine if they match. If the addresses do not match, the ESXi host drops the packet.