Monday, July 7, 2008

Ethernet Wiring and Loop Back

How to know what kind of network cable you have or need and how they are wired.

See pictures below for wiring diagram.

Working on point to point T1 connection issue today required making a ethernet loopback connector.

The loopback is made by cutting a normal RJ-45 cable and twisting the wires together that go to pins 1 & 4 and 2 & 5.

The link below is a great reference for ethernet cable wiring.

This is a pinout image from that site:

Update to post Sept 2008,
If you join Techrepublic you can get to another link with some good information regarding cabeling:

This image is from the Cisco link at the bottom of the post. This is how I identify cables. I hold both ends and look at the order of the cables as explained below.

---key information form the techrepublic link---

While you could put different ends on a UTP cable, typically it will have a RJ45 end with 8 pins.

With a normal Ethernet cable, the wires run straight through, from one end to the other. Straight-through cables are used to connect a PC to a switch

With a crossover cable, the source and destination of the UTP wires are crossed. This allows you to use it to connect a PC to PC, switch to switch, or router to router.

Cisco console and AUX port cabling

There are a few differences between Cisco cabling and other network device cabling. Two things immediately come to mind:

Cisco routers, switches, and firewalls use a special “rolled” cable for console and auxiliary port access.

Cisco offers intelligent serial cabling.

One of the most confusing things to Cisco newcomers is the concept of the console cable. Other SMB and home-networking devices don’t usually have a console port. With those devices, they receive a DHCP IP address and then you can configure them over the network from there. With Cisco devices, there is no IP address on the device, and you must first use the console port and console cable to configure the router, switch, or firewall OOB (out of band).

The Cisco console cable is a special cable. It isn’t wired like an Ethernet cable. However, if you didn’t have a console cable, you could cut off the end of a straight-through Ethernet cable, change the pin out, and recrimp it to make it a console cable.

Below, you can see the pin out of a console cable. The console cable is a “rolled” cable, because if you look at the pins from one end to the other, it is as if the end was rolled over (the order is flipped),

Traditionally Cisco console cables were RJ45-RJ45 and then you would use a RJ45-DB9 adaptor to connect it to your PC’s serial port (COM port). Today, new Cisco devices come with console cables that have a DB9 adaptor integrated/molded to the cable on one end (Figure D). Keep in mind that the data moving across the console cable is serial data (not Ethernet).

More info on these Cisco cables at:

---End of info from Techrepublic link---


sheila olson said...

Thank you for providing the information. I would like to see some more blogs on this topic.
Stream Line Voice & Data

hemmymission said...

Sometime few educational blogs become very helpful while getting relevant and new information related to your targeted area. As I found this blog and appreciate the information delivered to my database.