We can use ganglia to monitor metrics on servers, but it does not seem to be an option for network gear monitoring.
To monitor bandwidth of a switch, we need to either poll the port through SNMP protocol or somehow talk to sFlow/netflow agent running on the the switch/router. Any open source based option on the table? MRTG sounds a good out of box solution option, but I really wish I could use nagios or ganglia if possible because I am using them right now anyway (and I hope not to introduce another thing into the equation)