Ethernet Cabling Types
Type of Cable Pinouts Key Pins Connected
Straight-through T568A (both ends) or T568B (both ends) 1 – 1; 2 – 2; 3 – 3; 6 – 6
Cross-over T568A on one end, T568B on the other 1 – 3; 2 – 6; 3 – 1; 6 – 2
Many Ethernet standards use two twisted pairs, with one pair being used for transmission in each
direction. For instance, a PC network interface card (NIC) transmits on pair 1,2 and receives on
pair 3,6; switch ports do the opposite. So, a straight-through cable works well, connecting pair 1,2
on the PC (PC transmit pair) to the switch port’s pair 1,2, on which the switch receives. When the
two devices on the ends of the cable both transmit using the same pins, a cross-over cable is
required. For instance, if two connected switches send using the pair at pins 3,6 and receive on
pins 1,2, then the cable needs to connect the pair at 3,6 on one end to pins 1,2 at the other end, and
vice versa.
Cisco also supports a switch feature that lets the switch figure out if the wrong cable is installed:
Auto-MDIX (automatic medium-dependent interface crossover) detects the wrong cable and
causes the switch to swap the pair it uses for transmitting and receiving, which solves the cabling
problem. (As of publication, this feature is not supported on all Cisco switch models.)
NOTE Cross-over cables can also be used between a pair of PCs, swapping the transmit pair
on one end (1,2) with the receive pins at the other end (3,6).