SNMP

SNMP means Simple Network Management Protocol. It is a protocol that allows the querying of data from a network device such as a switch or router. It also allows traps which are the opposite of a query in that network devices contact a single host that collects these for further analysis or generating an alarm for the network operators.

One use that's common is MRTG to generate a nice graph of something such as bits per second through an interface or aggregate.

The above image is daily aggregate traffic of May 30/2008 at the Toronto Internet Exchange Point.

For more information on SNMP consult RFC 3417.