MAB is a fallback authentication method for devices that do not support 802.1X. When MAB is enabled on a switchport, the switch will first try 802.1X and if it fails, it will use the MAC address of the device as the username and password to authenticate it with a RADIUS server. The RADIUS server must have a database of MAC addresses that are allowed on the network. MAB can also support dynamic VLAN assignment and ACLs from the RADIUS server. MAB is not a very secure method because MAC addresses can be easily spoofed or changed. Therefore, MAB should be used with caution and only for devices that cannot use 802.1X. References :=

Some possible references are:

Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 3: Securing Networks with Cisco Firepower Next Generation Firewall, Lesson 3.2: Deploying Cisco Firepower Next-Generation Firewall, Topic 3.2.2: 802.1X and MAB

MAC Authentication Bypass Deployment Guide, Cisco, MAB Overview, What is MAB?

MAC Authentication Bypass (MAB), NetworkLessons.com, How MAB Works

MAC-based authentication (MAB), RADIUSaaS Docs, How it works

MAC authentication and username/password, Cisco Community, Answer by hslai