The security policy of a virtual switch includes a MAC address changes option. This option affects traffic that a virtual machine receives.
When the Mac address changes option is set to Accept, ESXi accepts requests to change the effective MAC address to a different address than the initial MAC address.
When the Mac address changes option is set to Reject, ESXi does not honor requests to change the effective MAC address to a different address than the initial MAC address. This setting protects the host against MAC impersonation. The port that the virtual machine adapter used to send the request is disabled and the virtual machine adapter does not receive any more frames until the effective MAC address matches the initial MAC address. The guest operating system does not detect that the MAC address change request was not honored.
In some situations, you might have a legitimate need for more than one adapter to have the same MAC address on a network—for example, if you are using Microsoft Network Load Balancing in unicast mode. When Microsoft Network Load Balancing is used in the standard multicast mode, adapters do not share MAC addresses.