The setting for the MAC Address Changes option affects traffic that a virtual machine receives.

When the option is set to Accept, ESX accepts requests to change the effective MAC address to other than the initial MAC address.

When the option is set to Reject, ESX does not honor requests to change the effective MAC address to anything other than the initial MAC address, which protects the host against MAC impersonation. The port that the virtual adapter used to send the request is disabled and the virtual adapter does not receive any more frames until it changes the effective MAC address to match the initial MAC address. The guest operating system does not detect that the MAC address change was not honored.

Note

The iSCSI initiator relies on being able to get MAC address changes from certain types of storage. If you are using ESX iSCSI and have iSCSI storage, set the MAC Address Changes option to Accept.

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.