In a real-life network, your serial interfaces will almost certainly be configured as DTE interfaces. Recall that a CSU/DSU usually handles the clocking for a synchronous serial interface. If you’re working in your own lab, however, you may be connecting the serial ports of two routers directly using what is known as a DCE-to-DTE crossover cable. These cables allow you to simulate a serial WAN connection without requiring a CSU/DSU or similar device.
Figure: Connecting the serial ports of two routers using a DCE to DTE crossover cable.
The main issue with connecting your serial interfaces in this manner is the fact that one of the devices will need to be configured as DCE in order to provide the timing mechanism required. The DCE-to-DTE crossover cable will have two different DB-60 interfaces – one marked DTE, and the other marked DCE. The router connected to the DCE end of the cable will need its serial interface configured as a DCE device.
For the sake of argument, we’re going to assume that we are simulating a WAN link between two routers, Router1 and Router2. Their serial interfaces have already been configured with relevant IP address settings. On the DCE device, we need to configure the serial interface using the clock rate command.
Router2(config-if)#clock rate 56000
%Error: This command applies only to DCE interfaces
Notice what just happened. I accidentally attempted to configure the clock rate on Router2 (the DTE device) instead of Router1. Router2 makes me aware of my mistake by letting me know that I have attempted to configure the wrong device – it recognizes its cable connection as DTE. Attempting the configuration again on Router1 yields the following results.
Router1(config-if)#clock rate 56000
Router1(config-if)#
Because we didn’t get an error message, the command completed successfully. Notice that we set the clock rate to 56000, a value represented in bits per second. A variety of clock rates are possible, and can be viewed by entering the question mark after the clock rate command.
If you’re looking for a quick way to check whether a serial interface is connected to the DCE or DTE end of a crossover cable, use the show controllers command. The first few lines of output are shown below.
Router2#show controllers serial 0
HD unit 0, idb = 0x710CC0, driver structure at 0x716140
buffer size 1524 HD unit 0, V.35 DTE cable
