Cisco hardware and software is extremely complex. When it comes to routers their abilities are unmatched which is why a lot of people strive for the Cisco certifications of CCNA, CCNP and the highly coveted CCIE. But who can afford to purchase 5 to 10 thousand dollars for the hardware that you would need in order to practice for those exams?
Cisco router emulators will the need just perfectly. While Cisco emulators have been around for a long time their success has been somewhat limited but their stability. Recently I was re-acquanted with a very good and very stable emulator named Dynagen.
The tool highly customizable, runs on Linux and Windows, can emulate several hardware platforms and allows you to add/remove hardware as needed. Perfect for testing and training.