Clock Rate on Cisco Serial Interfaces

The clock rate command is used only on a serial interface that has a DCE cable plugged into it. There must be a clock rate set on every serial link between routers. It does not matter which router has the DCE cable plugged into it or which interface the cable is plugged into. Serial 0 on one router can be plugged into Serial 1 on another router.

The clock rate command is set in bits per second. Besides looking at the cable end to check for a label of DCE or DTE, you can see if a router’s serial interface has a DCE cable connected with the show controllers int command. The new ISR routers automatically detect DCE connections and set the clock rate to 2000000. However, you still need to understand the clock rate command, even though the new routers set it for you automatically!