dynamips is an emulator of various Cisco platforms, that is licensed under GNU GPL, and runs under Windows, Linux, Solaris, MacOS, etc.
Dynamips started off as a MIPS emulator for Cisco 7200, and gradually ended up capable of emulating Cisco 7200 family, Cisco 3600 family, 2600 family (with some exceptions), and Cisco 3725 and 3745. Since it is a hardware emulator, it is bug for bug compatible with the real iron, and IOS on it would have the same bugs as on the physical hardware. Since it supports hypervisor mode, it is possible to run more then one router emulation on a single system, all connected through virtual network. Latest release candidates support packet capture on the virtual interfaces between the routers.
Performance of the emulator is not that great (1 or 2K packers per second, compared to 100s of kpps that actual hardware supports), but it is useful in testing configurations, preparing for Cisco certifications, debugging IOS, etc. I found it while reading up on IOS security, but there are people in both Cisco TAC and preparing/passing CCIE exams, that indicated in 7200emu formus that they use dynamips.
Current PC with a Gig or two of RAM can support a dozen or so router instances.
Based on the information from the developer, we should not expect switch emulation support in the forseeable future, since switches use custom ASICs, so while the main CPUs (MIPS or PPC) that the switches use, are supported, it is very tricky to emulate the power on self-tests of the ASICs (sending packets over loopback, etc), that switches attempt before declaring themselves functional. However 7200 is a bitchin’ platform for pretty much anything, capable of running latest and greatest IOS.
Blog of the author, where newest release candidates of the software are announced. Best place to check to see what bugs got fixed, and what line cards got supported in the latest release.
Forums/Discussion Board for c7200emu, that is moderated by the software’s author.
c7200emu – dynamips project page, detailing more or less up to date list of supported platforms.
Dynagen a dynamips configuration front-end, that allows one easily configure and manage dynamips instances. Currently considered a must have companion to dynamips.
dynamips TODO list, that allowes you to see what the developer is thinking about improving.