Creating or making a loopback cable/adapter is pretty simple. You need:

• Crimper
• Cat-5 cable (30cm should be enough)
• Knife
• RJ-45 connectors

Remove the cable pairs from the from inside the cat-5 cable. You need a single pair per loopback cable.

Next seperate the 2 wires at each end of the cable enough to slide into to RJ-45 connector.

The pinout is as follows:

Pin 1 -> Pin 3

Pin 2 -> Pin 6

When using with a cisco switch you may see a message that reads:

01:59:12: %ETHCNTR-3-LOOP_BACK_DETECTED: Keepalive packet loop-back detected on FastEthernet0/12.
01:59:12: %PM-4-ERR_DISABLE: loopback error detected on Fa0/12, putting Fa0/12 in err-disable state

To get around this issue the command no keepalives on the interface:

SW1(config)#int fa0/12
SW1(config-if)#no keepalive