Actually If I change the clockrate and define the bandwidth (less and High) what will happen serail interface???
can u suggest...
clockrate will be the actual speed used on the interface. If you configure a clockrate of 128kbps, and bandwidth of 256kbps - 128kbps will be your limit on that interface and routing protocols as well as QoS will see the interface as a 256kbps line.
It's a recommendation to match both values but the clockrate is the one that dictates the speed of the line. Bandwidth is there to inform QoS and routing protocols how fast the link is.
Additionally, if you don't enter a bandwidth information under the serial interface - it defaults to a T1 speed.