Auto-Negotiation Required for Auto-MDIX

Auto-MDIX allows connectivity between Cisco devices (and other vendors that support the HP Auto-MDIX standard) with a straight-through cable. In IOS version 12.2.18 and higher, Auto-MDIX is enabled by default on all interfaces. It is however, a requirement of Auto-MDIX, that auto-negotiation be enabled. This means that Auto-MDIX will be disabled on any interface were the duplex mode or line speed is set explicitly. For example, on a trunk where the switches are connected with a straight-through cable, issuing the following command switch#(config-if) speed 100 will shut down the trunk.

Another point worth mentioning, while Auto-MDIX is enabled on IOS versions 12.2.18 and higher by default, it does not show up on the interface config when it is enabled even if it is explicitly set on the interface.