What is auto-MDIX and why is it useful?

Auto-MDIX (Automatic Medium-Dependent Interface Crossover) detects whether a straight-through or crossover cable is plugged in and flips the port's internal wiring to match — so either cable works.

* Crossover wiring that auto-MDIX makes unnecessary. — W Nowicki, CC BY-SA 3.0, via Wikimedia Commons. *

Without auto-MDIX, you must use the correct cable type:

• Straight-through: Switch to PC, server, router
• Crossover: Switch to switch, switch to repeater

With auto-MDIX, either cable type works — the interface automatically adjusts the internal wiring.

Configuration:

S1(config-if)# mdix auto

Requirements for auto-MDIX:

• Interface speed and duplex must be set to auto (both must be auto for auto-MDIX to work correctly)
• Enabled by default on Catalyst 2960 and 3560
• Not available on older Catalyst 2950 and 3550

Verify auto-MDIX status:

S1# show controllers ethernet-controller phy | include Auto-MDIX

Tip: In modern networks, auto-MDIX means you rarely need to worry about straight-through vs. crossover cables. But on older equipment, cable type still matters.

Go deeper:

• Medium-dependent interface (Wikipedia) — the "Automatic MDI-X" section explains automatic crossover detection, eliminating crossover cables.