To configure the operating speed of a serial interface, which command should be used?

The command "clock rate bits-per-second" is the correct choice for configuring the operating speed of a serial interface, especially on a router's interface. This command is used in the interface configuration mode and specifies the clock rate for a serial link that operates in DCE (Data Communication Equipment) mode. In a serial connection, one end must provide the clock signal for correct timing, and this is where the clock rate is defined.

When configuring a serial interface, it’s important to recognize that only the DCE end needs to have the clock rate set. The command establishes the frequency of the bits being transmitted across the interface, ensuring that data is serialized correctly and received accurately on the opposite end.

The other commands listed do not exist in standard Cisco configurations for adjusting the speed of serial interfaces, which is why they are not suitable for this purpose. Understanding the role of DCE and DTE (Data Terminal Equipment) devices in serial communications also reinforces the necessity of using the correct command to ensure reliable network operations.