what is the differnece b/w clock rate and bandwidth?why do we set both?

Answer 1

I am assuming you are asking this question in the context of router configuration, such as connecting 2 Cisco routers back to back with a serial crossover cable for example.

In this scenario one router is a DTE and the other is DCE. You would end up setting the clock rate on one, and set a bandwidth on both. The clock rate determines the actual transmission speed for signaling on the wire, the router with the DCE cable gets the "clock rate" command, and this also sets the speed for the router on the other end.

If you don't know which router has the DCE interface, just use the "show controller" command and it'll tell you what's connected.

The "bandwidth" command does NOT specify the transmission speed, this is only used for reference by routing protocols when determining how to assign a metric to an interface.