This tutorial presents the cabling guide for the serial and console interfaces of the Cisco devices. Learn what the DCE and DTE devices are and how to change a serial interface into the DCE device.
Router
A router supports many interfaces. Among those the Serial, Ethernet and Console are the most common interfaces. Usually, every router has these interfaces. In this part, we will understand how the Serial and Console connections work. We will discuss the Ethernet connection in the next part of this article.
Serial interface
A Serial interface connects the router to the WAN network. In the WAN network, communication takes place between two types of devices; DTE and DCE.
DTE
DTE stands for the Data Terminal Equipment. The device which ends the communication line is known as the DTE device. It converts the data stream into the signals and vice versa.
DCE
DCE stands for Data Communication Equipment. The device which works in the middle of the communication line and provides a path for communication is known as the DCE device.
Let’s take a simple example. Suppose, you want to access a website. For this, you connect the computer with the Internet through the modem. A modem connects PC to the ISP. Further, ISP makes a connection with the hosting server.
The following image shows this example.
In this example:-
- PC and Server are the DTE devices, as they terminate the communication line at both ends respectively.
- Modem and ISP are the DCE devices, as they create a path for communication.
In communication, the DCE device provides Clock Rate and Bandwidth. The Clock rate is the speed of data transmission. The DTE device receives the values of Clock rate and Bandwidth from the DCE device.
A serial interface of the router can work both as DCE and DTE. By default, it is configured to work as the DTE interface.
In the job environment, when you connect a serial cable to the serial interface of a router, most probably the other end of that cable would be connected to a CSU/DSU device. A CSU/DSU (Channel Service Unit/Data Service Unit) device connects a router to the digital circuit (Wan Network). It works as a DCE device.
Since by default the serial interface of a router works as the DTE device and a CSU/DSU works as the DCE device, you hardly have to adjust the clocking in job life.
But in the lab environment, the situation is slightly different. To simulate a WAN network, usually, instructors connect two routers back to back on serial interfaces using a serial cable. Since by default all serial interfaces work as the DTE device, to make communication between two serial interfaces, one of them needs to be configured as the DCE device. To configure a serial interface as the DCE device, the "clock rate" and "bandwidth" commands are used from the interface mode.
The following image shows how these commands are used in the interface mode.
Connecting serial cable
Usually, a serial cable has a clear marking on both ends. The following image shows a serial cable with the marking.
If both ends are clearly marked, connect the DCE side to the DCE device and the DTE side to the DTE device. For example, if you want to connect a router and CSU/DSU, connect the DTE side and DCE side of the cable to the router and CSU/DSU respectively.
Similarly, if you want to connect two routers through the serial interfaces, connect the DCE side of the cable to the interface that you configured to work as a DCE device.
Knowing how the serial interface is configured
To know how a serial interface is configured and to know which side of cable attached to the interface, you can use the "show controller" command from the privileged exec mode.
The following image shows this command with the sample output.
The third line of the output tells whether the connected side of the cable is the DCE or DTE.
Console interface
The Console interface is used to connect a router or switch with the system for the management purpose. The following image shows the console interface from both switch and router.
To connect a PC with the console port, a special cable, known as the "rollover cable", is used. This cable is specially designed for the Cisco device. Besides this, it is also proprietary to Cisco. It means, it will not work for other types of connections. The following image shows a rollover cable.
A rollover cable is available in two variations; RJ-45–to–DB-9 and RJ-45–to–DB-25. These variations give us the flexibility to connect the cable on any available COM port. Basically, there are 2 types of COM ports; 9 pins COM port and 25 pins COM port. If your system has 9 pins COM port, select the RJ-45–to–DB-9 cable. Or if your PC has 25 pins COM port, select the RJ-45–to–DB-25 cable.
To connect a Cisco router or switch with the PC, connect the RJ-45 end to the console port and the other end into the COM port of your PC.
Once both are connected, use any standard terminal emulation program such as the PuTTY and Tera Term to configure the connected Cisco device.
That’s all for this tutorial. In the next part of this tutorial, we will discuss Ethernet connections in detail. If you like this tutorial, please don’t forget to share it with friends through your favorite social channel.
