When engaging in some type of lower-level troubleshooting, a loopback plug can be very useful for testing physical ports. Loopback plugs can work with diagnostic tools that are specifically built to work this way or they can be used to trick the application into sending data out just to see its response to data that comes back through the loopback. They are also used when trying to troubleshoot hardware components (e.g., ports) to make sure that the data is going out and coming back in without problems.
Loopback plugs function by sending data out of a connection and then looping it right back in. The amount and type of data received matches exactly the amount and type of data sent. Loopback plugs can be used for any type of connection, for example:
- Ethernet
- Fiber optic
- T1/E1
- Serial connections
On an Ethernet interface, in order to be able to receive the exact same information sent out, you can use a single RJ45 jack with the following connections, as shown in Figure 22.9 below:
- Pin 1 connected to pin 3
- Pin 2 connected to pin 6
- Pin 4 connected to pin 7
- Pin 5 connected to pin 8
Figure 22.9 – RJ45 Loopback Plug (Pin Sequence)