ProCurve - MDI and MDIX

MDI  MDIX

Copper ports on the switch can automatically detect the type of cable configuration (MDI or MDI-X) on a connected device and adjust to operate appropriately.   This means you can use a “straight-through” twisted-pair cable or a “crossover” twisted-pair cable for any of the connections—the port makes the necessary adjustments to accommodate either one for correct operation. The 10/100/1000Base-T port types on your switch support the IEEE 802.3ab standard, which includes the “Auto MDI/MDI-X” feature:  

Auto-MDIX (default): Senses speed and negotiates with the port at the other end of the link for port  operation (MDI-X or MDI). To see what the switch negotiates for the Auto setting, use the CLI show interfaces brief command or the “3. Port Status” option under “1. Status and Counters” in the menu interface.

MDI: Sets the port to connect with a PC using a crossover cable (Manual mode—applies only to copper port switches using twisted-pair copper Ethernet cables)

MDIX: Sets the port to connect with a PC using a straight-through cable (Manual mode—applies only to copper port switches using twisted-pair copper Ethernet cables)

Additionally, ProCurve Auto-MDIX supports operation in forced speed and duplex modes.  This means if you hard-code both sides to 100FD, then the MDI/MDIX negotiation will still take place.  Refer to the IEEE 802.3ab Standard Reference for further information.

Straight through Ethernet Cables

Now this is all fine and good when you actually are using a cross-over cable, or connecting to an archaic device that doesn’t understand Auto MDI/MDI-X, ,but what if a straight through cable is used where both link partners are set to Auto-MDI/MDI-X?

When a link establishes between two link partners, the auto-negotiation signaling between two devices is a “coin toss”.   There is really no telling which side will come up MDI and which will be MDIX.  MDI is considered the “master” and MDIX is considered the “slave”.    This signaling happens automatically on link establishment with auto-negotiation set.

If you set the switch to be MDI then it saves a bit of overhead between the switch port and the server NIC bouncing back and forth between who is master and who is slave before settling.  If the switch says “I am MDI” then things settle faster.  Some NIC vendors have the ability to set hard code the MDI status for the NIC as well.  Same logic holds true for the NIC.  

On the switch 

5406(config)# int a1 mdix-mode

 mdi                   Configures port for connecting a PC with a crossover cable

 mdix                  Configures port for connecting a PC with a straight-through cable

 automdix           Configures port for automatic detection of the cable

In other cases, some improvements have been seen when you tell either the client or the switch to be the MDI always.