Dynamips is very good, but can be frustrating to set up.
I use GNS3, which is a graphical front-end for Dynamips (which is itself a front-end for Dynagen).
http://www.gns3.net
Unfortunately Dynagen/Dynamips/GNS3 do not support emulation of Cisco switches, but they all support Cisco routers.
You need a real IOS image from a real router, and the simulation only supports 1700, 2600, 3600, 3700, and 7200 series routers. The latest version also supports PIX firewall simulation.
It's also possible to connect your virtual setup to a real network using the network card in your PC, but I've never quite managed to get this working.
You will need a fast PC, with a fair amount of memory - you can expect this to run rather slowly even if you're only simulating a couple of routers.
Also, if you plan on simulating 2600-series routers, you'll need to uncompress the IOS image as the simulation will crash if you try to use a standard compressed image. It uses RAR compression, so you'll need something like WinRAR to uncompress it (this is quick and easy, and only needs to be done once).
Hope this helps!
Kind regards,
Graeme
