Cacti is an open-source network graphing tool built on RRDtool. It polls routers, switches, and servers over SNMP and turns the results into bandwidth, CPU, and other performance graphs.
This nixCraft guide is the one I use for installing and configuring Cacti on Red Hat and CentOS, step by step:
nixCraft: Install Cacti Monitoring (RRD) Software on Fedora / RHEL / CentOS
Related: installing ntop on CentOS for per-host traffic analysis.