Solarwinds,Concord & MRTG are very products you could use.Concord is very expensive.MRTG (Multi Router Traffic Grapher) is the most flexible one.Good luck !
you don't need to change mode in this casefirst, make sure there are no other span session runningswitch(enable)set span disable switch(enable)set span inpkts enableexample : set span 3/1 3/2 inpkts enablethere are also other option you could use....
