Hope this helps:
Auto-MDIX
Until recently, certain cable types (straight-through or crossover) were required when connecting devices. Switch-to-switch or switch-to-router connections required using different Ethernet cables. Using the automatic medium-dependent interface crossover (auto-MDIX) feature on an interface eliminates this problem. When auto-MDIX is enabled, the interface automatically detects the required cable connection type (straight-through or crossover) and configures the connection appropriately. When connecting to switches without the auto-MDIX feature, straight-through cables must be used to connect to devices such as servers, workstations, or routers and crossover cables must be used to connect to other switches or repeaters.
With auto-MDIX enabled, either type of cable can be used to connect to other devices, and the interface automatically adjusts to communicate successfully. On newer Cisco routers and switches, the mdix auto interface configuration mode command enables the feature. When using auto-MDIX on an interface, the interface speed and duplex must be set to auto so that the feature operates correctly.
Note: The auto-MDIX feature is enabled by default on Catalyst 2960 and Catalyst 3560 switches, but is not available on the older Catalyst 2950 and Catalyst 3550 switches.
To examine the auto-MDIX setting for a specific interface, use the show controllers ethernet-controller command with the phy keyword. To limit the output to lines referencing auto-MDIX, use the include Auto-MDIX filter.
