Cisco VIRL is out ... for a price
Just wanted to put this up has Cisco has recently released VIRL. This is a network simulation platform that you can run Cisco operating systems on which are the same operating systems that run on Cisco hardware so anything you do in this simulation environment would be realistic if it was running on dedicated hardware. You would be able to test your changes before throwing them in production. Some things that are different between VIRL and CML (Cisco Modeling Labs)