[conspire] (forw) Re: [TAG] Wiring a house with ethernet

Dan Martinez dfm at razorwind.org
Thu Oct 20 15:54:28 PDT 2005

Peter Knaggs wrote:

> All of these fascinating wiring discussions
> reminded me of one cable I was trying to use,
> and I was puzzled why it didn't work for
> gigabit but was OK for regular 10 and 100 base-T ethernet.
> I was trying to use it for connecting from a
> machine with Broadcom BCM5751 gigabit to a
> Dell PowerConnect 2324 switch. The switch
> has two gigabit ports on it.


> So any ideas why it doesn't work at all
> for gigabit ethernet?

10/100BaseT Ethernet uses only four of the eight conductors in the
cable. Gigabit Ethernet uses all eight.

Some older crossover cables swap *only* the two conductor pairs used
by 10/100BaseT, and leave the other two unchanged. Combine this with
the fact that many newer switches automatically detect and adjust for
the presence of crossover cables, and you have the potential for a
situation exactly like the one you describe.

One of these old-style crossover cables would be perfectly suitable
for 10/100 megabit traffic when used with an auto-sensing switch, but
would be rejected as unsuitable for gigabit traffic when used with the
same switch.

There's no guarantee that this is the actual cause of the behavior
you're seeing, of course, but it's the best guess I can come up with
based upon your description. If you can see it through the plug, the
color sequence of the wires on both ends of the cable might shed some
more light upon its exact nature.

More background regarding crossover cables and gigabit ethernet can be
found here:


(I'm a software type by day. No, really. I swear.)


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