When must a router serial interface be configured with the clock rate command?

When you are connecting a cable to the serial interface of the router, there is a need to do clocking which can be done with the use of an external device. Some examples of external devices that can be used are CSU and DSU. This will make it easier for the interface device to be connected to a digital circuit.

Take note that the serial cable is different from the ethernet port. There are some people who assume that these two things are the same. The ethernet port will allow different devices to be connected to the computer without any issues.