The setting for the Forged Transmits option affects traffic that is transmitted from a virtual machine.

When the option is set to Accept, ESX  does not compare source and effective MAC addresses.

To protect against MAC impersonation, you can set this option to Reject. If you do, the host compares the source MAC address being transmitted by the operating system with the effective MAC address for its adapter to see if they match. If the addresses do not match, ESX drops the packet.

The guest operating system does not detect that its virtual network adapter cannot send packets by using the impersonated MAC address. The ESX host intercepts any packets with impersonated addresses before they are delivered, and the guest operating system might assume that the packets are dropped.