MRTG is a powerful tool used for monitoring the traffic load on network links(router,switch…).In other word , it is a web based application used to monitor and graphically display traffic and usage or information of a device.
It generates HTML pages containing PNG images with visual representation of the traffic. It can produce daily, weekly, monthly, and yearly graphs. In addition to monitoring , it can also generate graphs based on the output of any application, for example, SIGNAL STRENGTH of a modem(Eb/No), Voltage level of UPS(input /output), memory usage etc.
MRTG is written in perl and works on most of the operating systems ,including most Microsoft Windows versions,UNIX/Linux.
if you have a router and you want to monitor the traffic on its interfaces , you can do it by simply running the MRTG on a server and monitoring the particular interface.
A Note !
To get the correct output, you should configure/install the SNMP and should have the rights/authority to monitor the traffic on that device.