As shown in the figure above, we use a straight-through cable in between the PC (MDI type) and switch (MDIX type). A crossover cable is required when we have the same type at both ends, such as PC-to-PC or switch-to-switch connection. If the device supports Auto MDIX, it changes the port type itself according to the other side, and no crossover cables are required. The exact name of this switching feature is Auto MDIX (Medium dependent interface crossover).
We MUST enable speed and duplex auto-negotiation as a prerequisite. We have to type the following commands in a Cisco switch to enable it.
It is enabled by default on the latest switches and IOSes if supported. Unfortunately, Auto MDIX is not supported by all Cisco Catalyst switches (listed at the bottom of the page).
Note(2020): Today, almost all brands and models support Auto MDIX, including PCs and end devices.
Q1) Which pins are used on MDI &MDIX type ports?
MDI : Pins:1 , 2 transmit 3 , 6 receive
MDIX : Pins:1 , 2 receive 3 , 6 transmit
Q2) Does it work if I use a crossover cable with an Auto MDIX port?
Yes, it does. Both straight-through and crossover cables work well.
Q3) Can I fix speed & duplex and use Auto MDIX?
No. Auto MDIX uses the speed negotiation process to recognize the port type of the opposite side. You need to use a crossover cable if you have to fix speed and duplex.
Q4) Do I need Auto MDIX on both interfaces to let it works?
No. It is sufficient if just one interface supports Auto MDIX, but speed and duplex must be auto on both interfaces.
Q5) How can I check if my switch interface supports Auto MDIX?
You can check it by using the show interface capabilities command and look at the Auto-MDIX line.
Q6) How can I check the Auto MDIX status in my switch interface?
Type the show interface command, find the line below. First on/off indicates whether Auto MDIX is enabled. The second on/off shows if you have speed/duplex auto.
Auto-MDIX on (operational: on)
Q7) Is it possible to see if the interface already been crossed by Auto-MDIX?
No, unfortunately, It is not possible.
Q8) How to set Auto-MDIX on HP Switches?
You can use interface <InterfaceName> mdix-mode [ auto-mdix | mdi | mdix ] command to select the MDIX mode. As you can see, you are also able to fix the port type manually.
Q9) What about Juniper Switches?
Auto MDIX is enabled by default in Juniper Switches. You can disable it by issuing no-auto-mdix; command. If the interface is 10 Gig, you can manually select interface type with mdi-mode mdi; or mdi-mode mdix; commands. Some Juniper switches with proper Junos can support auto MDIX without speed auto-negotiation.
Cisco Switches without Auto MDIX support
Cisco Switches with Auto MDIX support
Cisco IE 3000