From my understanding when this is set to 802.1x & MAC. The port will first check the device to be 802.1x compliant, If it is not then it will try to authenticate by using the MAC of the device as the username and the password. If the MAC address has been placed in the RADIUS database, then it will be able to authenticate. 
Hi! I'm using "Port Based Authentication" on my PowerConnect 2848, the RADIUS server is Windows Server 2008 R2. The workstations authenticated correctly and everything works fine.  For my VoIP devices that doesn't support RADIUS, I would like to add MAC filtering. Under Port Based Authentication -> Choosing Port -> Authentication Type, there are 3 options, "802.1x Only", "MAC Only" and "802.1x & MAC". 1. What is the different between MAC only and MAC & 802.1x ? 2. As far as I know, when using MAC option in 802.1x, the switch send to the RADIUS server an authentication request with the MAC address as the username, and generate some password so I can create usernames in the RADIUS server with this information and filter like that just the MAC addresses I approve. The question is, what is the password the 2848 sending to the RADIUS server in this situation, is it using a pre-configured password for all MAC authentications? or is it using the MAC address as the password in addition to the username field?  And any additional information about what I'm trying to accomplish would be great, Many thanks!
802.1x by MAC address with Power Connect 2848