Cabling Cisco Devices Guide

This tutorial explains what DCE and DTE devices are, how to convert a serial interface into a DCE device, and which cables to use to connect to serial and console interfaces.

Router

A router supports many interfaces. Serial, Ethernet, and Console are essential and generally available on all routers. A serial interface connects it to another router or a WAN network. An Ethernet interface connects it to a local network or another router. A console port connects it to a PC.

Serial interface

A Serial interface connects the router to another router or a WAN network. In a WAN network, communication occurs between two device types: DTE and DCE.

DTE

DTE stands for the Data Terminal Equipment. The device that 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 that works in the middle of the communication line and provides a path for communication is known as the DCE device.

Let's take an example. Suppose you want to access a website on your PC. For this, you connect your PC to the Internet through a modem. The modem connects the PC to the ISP. Further, the ISP makes a connection with the hosting server. The following image shows this example.

In this example:-

• PC and Server are DTE devices because they terminate the communication line at both ends.
• The modem and ISP are DCE devices, as they provide a path for communication.

In communication, the DCE device provides Clock Rate and Bandwidth. The Clock rate is the speed of data transmission. Bandwidth is the amount of data it can transfer in a given time. The DTE device receives the Clock rate and Bandwidth values from the DCE device. The router's serial interface can operate as both DCE and DTE. By default, it works as the DTE interface.

In a production environment, when you connect a serial cable to a router's serial interface, the other end of that cable is most likely 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 operates as the DTE device and a CSU/DSU as the DCE device, you hardly have to adjust the clocking on the job.

However, in a lab environment, the situation is slightly different. To simulate a WAN network, usually, you connect two routers back-to-back on serial interfaces using a serial cable. By default, all serial interfaces operate as DTE devices. To establish communication between two serial interfaces, one of them must be configured as the DCE device. To configure a serial interface as the DCE device, use the "clock rate" and "bandwidth" commands from the interface mode. The following image shows an example of these commands.

Connecting the 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 via the serial interfaces, connect the DCE end of the cable to the interface you configured as a DCE device.

The show controller command

To know how a serial interface is operating and which side of the cable is attached to the interface, you can use the "show controller" command from the privileged exec mode. The following image shows this command with the output.

The third line of the output indicates whether the cable's connected side is DCE or DTE.

Console interface

A Console interface connects a router or switch to a PC for management. The following image shows console interfaces on a switch and router.

To connect a PC to the console port, a special cable known as the "rollover cable" is used. A rollover cable uses Cisco's proprietary technology and works only with Cisco devices for console connection. The following image shows a rollover cable.

A rollover cable is available in two variations: RJ45–to–DB9 and RJ45–to–DB25. These variations give you the flexibility to connect the cable to any available COM port. There are two types of COM ports: 9-pin and 25-pin. If your system has a 9-pin COM port, select the RJ45–to–DB9 cable. Or if it has a 25-pin COM port, select the RJ45–to–DB25 cable. To connect a Cisco router or switch to a PC, connect the RJ45 end to the console port and the other end to the PC's COM port.

Once both are connected, use any standard terminal emulation program, such as PuTTY or Tera Term, to configure the connected Cisco device.

Conclusion

Understanding the distinctions between DTE and DCE devices is crucial for effectively setting up and managing network communications. This tutorial outlined the essential components and terminology of cabling Cisco devices, including the roles of the serial and console interfaces. By knowing how to identify and configure these interfaces, along with the appropriate cables required for connections, you can ensure reliable communication in both production and lab environments.