The MAC Address Change Policy is set to "Accept" for virtual switches.

Each virtual NIC in a virtual machine has an initial MAC address assigned when the virtual adapter is created. Each virtual adapter also has an effective MAC address that filters out incoming network traffic with a destination MAC address different from the effective MAC address. A virtual adapter’s effective MAC address and initial MAC address are the same when they are initially created. However, the virtual machine’s operating system may alter the effective MAC address to another value at any time. If the virtual machine operating system changes the 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 adapter authorized by the receiving network. System administrators can use virtual switch security profiles on ESX Server hosts to protect against this type of attack by setting two options on the virtual switches. These options are MAC Address Changes and Forged Transmits.

MAC address changes are set to accept by default meaning that the virtual switch accepts requests to change the effective MAC address. The MAC Address Changes option setting affects traffic received by a virtual machine. To protect against MAC impersonation this option will be set to reject, ensuring the virtual switch does not honor requests to change the effective MAC address to anything other than the initial MAC address. Setting this to reject disables the port that the virtual network adapter used to send the request. Therefore, the virtual network adapter does not receive any more frames until it configures the effective MAC address to match the initial MAC address. The guest operating system will not detect that the MAC address change has not been honored.