Why does 1000BASE-T need Auto-MDIX?

Question

A 100BASE-TX PC interface sends out its frames on pins 1,2 and receives frames on pins 3,6. If we connect this PC's NIC with that of another PC we cannot use a straightout cable, because the frames sent by the first PC would arrive on pins 1,2 of the second PC, which can only trasmit, not receive. In this case we clearly need a crossover cable or auto-mdix.

Now if we have a PC with a 1000BASE-T NIC, frames can be sent out and received on any of four cable pairs. If we connect this PC to another 1000BASE-T PC and the first one sends a message out of pins 1,2 then the second PC may use its 1,2 pair to receive the message. The way I see it, no crossover is necessary.

Where am I going wrong?

Answer 1

1000Base-T strictly speaking does not need auto-MIDX. However, when the standard was written, inter-operation with older technologies was taken into account. So to keep things simple, when communicating with a 100base or 10base device, auto-MIDX is there so crossover cables are no longer necessary.

Edit: I forgot to mention, while auto-MIDX is in the standard, it is an optional feature, so it may not be present in all hardware.

Answer 2

When is a crossover cable or Auto-MDIX necessary?

For switch to switch connections, traditionally these required an ethernet cross-over cable but Auto-MDIX in the 1000Base-T standard solves the problem without need for a cross-over cable (straight through is fine). If your laptop NIC supports Auto-MDIX, it also allows laptop to laptop connections with a straight through cable.