compaq loopback cables ?

[tg]

I recently bought a cisco module NM-16ESW (16 port switch) and when it
arrived all the sockets had little 2 inch cables sticking out of them.
These cables were sealed at the end and some were labelled:
Compaq
PN 317463-001
SPARE
317465-001
ID 10036233 REV. C
I saw on google they are called 'Compaq loopback cables' and I saw
several places that advertised them for sale but I don't know what
they're for.
does anyone know what the function of these is? thanks for any advice.

[Doug McIntyre]

Guess they wanted to get rid of them.. :)

I don't find them all that useful, but they just loop the transmits
back out to the receives for the port, so you can test that everything
looks good on the port and comes up and tests out for the switch.

Ie. make one here.

http://www.tikkiweb.net/loopback

Much like a T1 loopback connector which will loop the T1 transmit back
to receive so you can test the wire and find faults, you can test each
port with the ethernet loopback plugged in.

[Bo...@hotmail.co.uk]

Without researching the exact devices it seems likely
that they are Ethernet loopback plugs.

I have not read these but the GOOGLE heading seems
to suggest that they will be of interest.

www.juniper.net/techpubs/software/nog/nog-interfaces/html/fe-ge-loopback25.html

www.juniper.net/techpubs/software/nog/nog-interfaces/download/fe-ge-loopback2.pdf
"Use Loopback Testing for Fast Ethernet and
Gigabit Ethernet Interfaces"

They will bring the port UP if it is enabled.

Any packets sent out will be sent back in.

If STP is enabled then of course the ports
will transition to STP blocking state.
Unless of course portfast is enabled and
a broadcast storm overwhelms the
CPU first.