One other interface configuration command that you’ll need to be familiar with is the bandwidth command. The bandwidth command is used to specify the configured bandwidth of an interface, such as serial 0. It’s worth noting that this command doesn’t change the actual bandwidth of a physical interface, but instead communicates the bandwidth available on an interface to upper-layer protocols. For example, serial interfaces have a default bandwidth of 1.544 Mbps on a Cisco router. However, if the interface has only a 64 Kbps circuit attached, you would issue the bandwidth command on the interface specifying this. This number would then be used by certain routing protocols (such as IGRP) that use bandwidth as one of their metrics in determining the best route to a destination. While IGRP will be looked at in more detail in Chapter 8, for now it’s important to simply know how to configure the bandwidth value for a serial interface. The bandwidth command is always followed by a value expressed in kilobits per second (Kbps). This is important to remember, since it is easy to confuse the bandwidth and clock rate commands.
In the example above, the bandwidth on the interface S1 has been set to 64 Kbps.