Cisco SDM Simulator
I hope you enjoy this really cool SDM emulator from Cisco. It is free to download and use and will come in very handy for your CCNA exam studies. It isn’t connected to a live network but is set up for you to be able to use most of the commands you will need to know for the CCNA.
http://www.cisco.com/pcgi-bin/tablebuild.pl/sdm-tool-demo
Remember that there are some SDM videos on the site for you to refer to.