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.

Cabling Lab Topology

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.

Straight-Through Cable Pin Layout

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.

Crossover Cable Data Flow

When Gig Ethernet (1000-BaseT) is in use, all eight wires are used, and every wire can send and receive at the same time.

Straight-Through Gig Ethernet Cable Layout

Gig Ethernet crossover cables cross over thusly:

Crossover Cable Layout (Gig Ethernet)

You may never need crossover cables, though, thanks to…

Auto MDI-X

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.

Auto-MDIX In Action

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

Now it’s time for us to revisit the Ethernet header and trailer we examined in the previous tutorial.   And when you’re done there, visit my  CCNA Video Boot Camp on Udemy!

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