The security policy of a virtual switch includes a MAC address change option. This option affects the 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.
