CCNA / CCENT / Network+ Tutorial: Crossover Cables And Auto MDI-X
Now that we’ve taken a deep look at Ethernet standards and cabling, it’s time to put the different cable types you’ll see on your CCNA, CCENT, and Network+ exams into use. We have three separate physical connections in this lab, and we’ll need two different cable types to make it work.
For the host-to-switch connections, we’ll need straight-through cables, so named because the wires in the cable run straight from a particular pin at the host end to the same numbered pin at the switch end.
The switch-to-switch connection presents a special challenge, as each switch transmits on pins 3 & 6 and receives on the 1 & 2 pins. A straight-through cable won’t do here.
That challenge is met by the crossover cable, so named because the wires cross over from a particular pin on one end of the connection to a different pin at the other end. Either switch can send a frame on one of the “transmit” pins, and thanks to the crossover, that frame arrives on the remote switch on a “receive” pin.
When Gig Ethernet (1000-BaseT) is in use, all eight wires are used, and every wire can send and receive at the same time.
Gig Ethernet crossover cables cross over thusly:
You may never need crossover cables, though, thanks to…
What follows is something you should know for real-world networking, and it may well show up on your exam. However, unless Auto MDI-X is specifically mentioned in an exam question, go with the theory presented in this chapter regarding straight-through and crossover cables:
- Same devices = Crossover cable
- Not the same type of devices = Straight-through cable
With that in mind, let’s chat about Auto MDI-X.
Whether it’s for a home lab or a client site, needing a certain cable type and not having one is a terrible feeling – a feeling that can be cured by Auto MDI-X (Automatic Medium-Dependent Interface Crossover) This life-saving feature allows a device to dynamically adapt to a straight-through cable where a crossover cable would normally be required, and vice versa.
To enable Auto MDI-X on a Cisco switch port, set speed, duplex, and mdix to auto on that port.
SW1(config)#int fast 0/1 SW1(config-if)#speed auto SW1(config-if)#duplex auto SW1(config-if)#mdix ? auto Enable automatic MDI crossover detection on this interface SW1(config-if)#mdix auto