Automatic medium-dependent interface crossover (Auto-MDIX) is a feature that allows the switch interface to detect the required cable connection type (straight-through or crossover) and automatically configure the connection appropriately. With Auto-MDIX enabled, you can use either a straight-through or crossover type cable tin order o connect to the other device, and the interface automatically corrects for any incorrect cabling.
The 2950 does not support Auto-MDIX. The 2940, 2970 and 3750 only supports Auto-MDIX. Therefore, you need this support on the 6k side, where the module does not support it.
You can still configure the Auto-MDIX because the Cisco IOS has this feature, but it is limited to certain modules. If you try and configure it on a module where it is not supported, you get errors messages, which disappear only after you disable the MDIX.
The workaround is to disable the Auto-MDIX feature on the port.
In order to enable automatic media-dependent interface with crossover detection, use the mdix auto command. In order to turn automatic detection off, use the no form of this command.