Uplink Ports
Last updated: 5/16/2001

Q. What is an uplink port and what are the ways to connect two hubs/switches together?

A.  There is no big mystery about the difference between an uplink and a regular port.  Each Ethernet interface has two transmit pins + and - and two receive pins (the other pins may have wires running between them, but they are not used).  The transmit pins at one end of a cable have to be connected to the receive pins at the other end and vice versa. An uplink port does not crossover the transmit and receive pins and a regular port does.

If two hubs/switches (What is the difference between a hub and switch?) are connected together with a straight-thru cable then one end must crossover (regular port) and one end must not (uplink port).  If a crossover cable is used to connect them, then the ports at both ends must be the same kind of port.  If a straight-thru cable is used to connect them, then the ports must be different.  A PC can be connected to an uplink port with a crossover cable and to a regular port with a straight-thru cable.  Also, be aware that many hubs/switches share the uplink port with one of the regular ports, usually port 1.  Both ports will not work if they are both connected at the same time.  Finally, many hubs and switches have a switch associated with the uplink port that can switch the port between uplink and regular port configurations.

Also see Internet Connection Sharing FAQ