RRDTOOL is required by many network utilities, e.g cacti, nagios & many others to plot graphs.
By default rrdtool is not included in CentOS. You may need to install it for monitoring purpose. You can either use source to compile or install using rpm.
To install using rpm all you need is following two commands.
http://apt.sw.be/redhat/el5/en/i386/rpmforge/RPMS/rpmforge-release-0.5.2-2.el5.rf.i386.rpm
yum install rrdtool -y
once sucessfully installed, you’ll find rrdtool in /usr/bin/
thanks its work