This command is used to set the clock rate on serial links. While you can configure the clock rate on both ends of the serial link, the router will only accept the command on the DCE end.
Note: Clock rate sets how many bits can be sent over a given time. If the clock rate is set to 64000, you will never be able to send faster then 64 kbs over the interface.
Router(config-if)#clock rate <number>
In this example, we will set R1’s Serial1/1 interface to have a clock rate 64000