Gigabit ethernet over copper with a crossover cable (1000baseT)

You may have experienced not being able to connect two 1000baseT NICs or two 1000baseT switch interfaces, using a normal crossed ethernet cable. The reason you can't do this, is that 1000baseT use all four wire pairs available in a Cat5 cable, whereas 100baseTX only uses two wire pairs.

Because of this, the cable pinout is different.

Some smart 1000baseT interfaces (like the ones in Apple's G4 and G5 computers) don't need a crossover cable. They have something known as "auto-mdix", which enable them to sense how the cable is wired, and adjust to that.

But that is not the case for all interfaces.

A "normal" straight-trough Cat5 cable is wired like this (according to EIA 568-B):

PinConnector #1Connector #2
1white/orangewhite/orange
2orangeorange
3white/greenwhite/green
4blueblue
5white/bluewhite/blue
6greengreen
7white/brownwhite/brown
8brownbrown

The notation in the above table is based on cables where the strands are either a solid colour, or white with a thin stripe of colour. In other cables the "solid coloured" strands may be mostly a solid colour with a thin stripe or stippling of white..

If you grab the connector and hold it with the fastener pointing away from you, and the cable downwards, it looks like this (pin 1 is to the left):

an RJ-45 jack

Combining this with documentation for a 1000baseT GBIC, we get this pinout for a 1000baseT crossover Cat5 cable:

PinConnector #1Connector #2
1white/orangewhite/green
2orangegreen
3white/greenwhite/orange
4bluewhite/brown
5white/bluebrown
6greenorange
7white/brownblue
8brownwhite/blue

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Last updated: 2007-05-09 15:05