5.2.d Describe 802.1x

5.2.d [ii] MAC authentication bypass

The best and most secure solution to vulnerability at the access edge is to use the intelligence of the network. One access control technique that Cisco provides is called MAC Authentication Bypass (MAB). MAB uses the MAC address of a device to determine what kind of network access to provide. MAB enables port-based access control using the MAC address of the endpoint. A MAB-enabled port can be dynamically enabled or disabled based on the MAC address of the device that connects to it.

Prior to MAB, the endpoint’s identity is unknown and all traffic is blocked. The switch examines a single packet to learn and authenticate the source MAC address. After MAB succeeds, the endpoint’s identity is known and all traffic from that endpoint is allowed. The switch performs source MAC address filtering to help ensure that only the MAB-authenticated endpoint is allowed to send traffic.

By default, a MAB-enabled port allows only a single endpoint per port. Any additional MAC addresses seen on the port will cause a security violation. Multi-domain authentication was specifically designed to address the requirements of IP telephony. When multi-domain authentication is configured, two endpoints are allowed on the port: one in the voice VLAN and one in the data VLAN. Either, both, or none of the endpoints can be authenticated with MAB. Additional MAC addresses will trigger a security violation.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5445-5451).  . Kindle Edition.

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6638/config_guide_c17-663759.html

http://www.cisco.com/c/en/us/support/docs/lan-switching/8021x/98523-8021x-cat-layer3.html#MDA