Up Late Tinkering: Discovering CSU\DSU Loopback Plugs

In studying for the certification tests, mention was made in class, and in the books, about doing loopback tests to help in troubleshooting router WAN connections. They mentioned the required commands to enter at the router console, but no mention was made that anything needed to be done physically to the equipment.

One day I was in Leland, Mississippi trying to troubleshoot a downed T1 connection at the school district’s main office. The telco tested their circuits and said they were ok. Exasperated, I finally called Cisco’s tech support. The Cisco tech said that I needed to make a loopback plug, remove the T1 line from the

CSU\DSU, insert the plug into the DSU\DSU, and enter some commands at the router console. If the serial interface came “up and up”, then the router was good and the problem was most likely on the telco’s equipment. He also said that the telcos almost always say that it isn’t their problem. But if I tell them that my physical loopback worked, then they will admit that it’s their equipment, if it is.

So, I made the loopback plug in the manner in which he instructed me and tested the router. The interface came back up just as he said it would. So, I called the telco back and told them that my loopback tested good. They hemmed and hawed and then finally said they would do a loopback “again” on their end. Sure enough they “discovered” that they couldn’t loop their smartjack and that the problem was with their equipment at the CO (Central Office). Within 20 minutes, the school district’s router was working fine.

Cisco has a document on their web site that tells you how to make a loopback plug, but if you don’t know about it, you won’t go looking for it. Also, if you have no access to the Internet at the site because the line you are trying to fix is your connection to the Internet, how are you going to access Cisco’s web site? This is something I’d really wished was part of my training. From then on, I have always made sure that I had T1 and 56k loopback plugs in my laptop carrying case and they have saved my hide many, many, many times over.

How to Make CSU\DSU Loopback Plugs

Constructing loopback plugs for CSU\DSUs is pretty simple and straightforward. If you are accustomed to making Ethernet cables, then you should have little problems making a loopback plug. With the tip of the connector facing you, the clip on top and the wires extending out the back, the pins are numbered 1 through 8 reading from the left to the right

T1 Plug

Basically all you need is an RJ45 connector and two wires from a piece of scrap Ethernet cable. A length of about 4 inches should be more than sufficient.
For a T1 CSU\DSU, the idea is to connect pin 1 to pin 4, and pin 2 to pin 5. Insert the 2 wires into the connector as shown in Figure 1 and crimp.

Figure 1. T1 CSU\DSU loopback plug.

56K Plug

For a 56k CSU\DSU, the idea is to connect pin 1 to pin 7, and pin 2 to pin 8. Again, Insert the 2 wires into the connector as shown in Figure 2 and crimp.

Figure 2. 56k CSU\DSU loopback plug.

No Crimper Handy? No Problem.

So, what if you don't have a pair of crimpers handy?

Find a pre-made CAT 5 cable that has the connectors already installed on both ends.
Cut the cable in two about three inches from one of the ends.
Remove the shielding and separate the wires.
Strip the necessary wires leaving about a half-inch or so of bare metal on the ends and then twist the ends together.
For a T1 loopback plug, strip and twist wires 1 and 4 together and then strip and twist wires 2 and 5 together.
Leave the remaining wires dangling out the back. Do the same for a 56k loopback plug strip except twist wires 1 and 7 together and 2 and 8 together. See the diagram for a T1 loopback plug below.