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Ethernet RJ-45 LoopBack, Crossed and Pinouts

posted Oct 31, 2008, 1:16 PM by Erez Kalman   [ updated Dec 14, 2008, 1:49 PM ]
An ethernet loopback is used to... perform loopback tests - obviously...
Lets cut to the chase - you can create two types of loopbacks, most only require the basic type.

RJ-45 Pinouts for Ethernet:


Pin Name Description
1 TX_D1+ Tranceive data +
2 TX_D1- Tranceive data -
3 RX_D2+ Receive data +
4 BI_D3+ Bi-directional Data+
5 BI_D3- Bi-directional Data-
6 RX_D2- Receive data -
7 BI_D4+ Bi-directional Data+
8 BI_D4- Bi-directional Data-
We always look at the connector from the underside - connectors
away from us.

Basic loopback (shown below - image from Juniper):
  1. Looking from the underside of the connector, CAT5 cable connects from bottom.
  2. Connect a cable to pin 1 and 3.
  3. Connect a cable to pin 2 and 6.
  4. Crimp.










Full loopback (often needed for gigaports - image from I-Hacked)


  1. Looking from the underside of the connector, CAT5 cable connects from bottom.
  2. Connect a cable to pin 1 and 3.
  3. Connect a cable to pin 2 and 6.
  4. (So far - same as the basic loopback).
  5. Connect a cable to pin 4 and 7.
  6. Connect a cable to pin 5 and 8.
  7. Crimp.



















Crossed

When to use ? A crossover cable is used (generally) when connecting the same device type to one another, e.g. switch to switch/switch-hub/hub-hub (unless using the uplink port) / computer-computer / router lan-switch or hub, etc...
Many modern switch's, hub's and router's support automatic MDI/MDI-X (Medium Dependent Interface / Medium Dependent Interface crossed), In general computers and routers use MDI and Switchs and Hubs use MDIX.

Quick tip: When creating crossover/loopback you always cross the cables as follows:

Side A        Side B
1                   3
2                   6
3                   1
6                   2

Leaving the other cables to run without crossing them; an easy way to remember how to cross is 1,2,3,6 goes to 3,6,1,2 (one, two, three, six -> three, six, one, two)



Straight Cable



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