i am starting to investigate this as i want to emulate a network with switches and routers , to help me pass ICND1 CCNET , i see routersim by todd lammale i believe and something by a company called boson, and a free ware one gns3 which doesn't seem to work on my pc,,not really sure which is the best, want one that runs SDM if possible, and if you guys can make any recommendations on the ones I listed or ones i don't know about it certainly would be appreciated..Victor