In Ethernet networking, Twisted Pair connected to the transmitting pair on one end, it should connect to the receive pair on the other end, and vice versa. For the correct duplex setting; it is important to have the correct cable type defined for each port. Connections between particular devices, for example, switch-to-switch, switch-to-host, switch-to-router, router-to-router devices and router-to-host; required the use of exact cable types (crossover or straight-through).
Auto-MDIX is standing for Automatic medium-dependent interface crossover. Its function is to detect the Ethernet cable type on every port on the switch automatically. It also automatically adjust (straight-through or crossover); to make a link over that cable.
Most of Cisco switches support the MDIX auto interface configuration command in the CLI to enable the automatic medium-dependent interface crossover feature. When this feature enabled the switch automatically detects the type of cable attached; to the port and configures the interfaces accordingly. So, if this function enabled we can use either a crossover or a straight-through; cable for connections to a copper 10/100/1000 port on the switch, anyway; of the type of device on the other end of the connection. Most of the new switches by default enable the auto-MDIX feature.