Bandwidth utilization is polled by using an NMS (network monitoring system).

This is done by using SNMP and querying the MIBs on network devices.

If you are monitoring a single link and is using windows then you can use PRTG. If you are monitoring a couple of devices and interfaces then you can use the opensource alternative like mrtg or rrdtool.

Configure your router/switch to allow accept snmp queries.

In global configuration mode,

router>en

router#config t

router(config)#snmp-server community

router#end

router#write mem

Thanks,

k0rg