Serial Interface

There are many interfaces in a CISCO router. By default, serial interfaces of a CISCO router are all data terminal equipments (DTE) devices, so if you need it to act like a DCE device, these interface must be provided with clocking. Clock rate is defined on DCE interface.

Serial connection R1 to R2

Here, in the figure we configure clock rate on Se0/0/0 of R1 router. Clock rates are defined in bits per seconds. So,
syntax:
R1(config)# int s0/0/0
R1(config-f)# clock rate 64000      <==== (Clock rate is 1200 - 8000000)

To verify that the interface has a DCE Connection or not
R1# sh controllers s0/0/0