Dynamips: virtual Cisco
Check out a free and very powerful router emulator called Dynamips and its graphical front-end Dynagen(www.dynagen.org). It is not a simulator but a true emulator. By creating a suitable control file, and assuming you have legal copies of any required IOS images, you can boot multiple routers in a Windows machine or VM, connect them together virtually, and even attach their virtual interfaces to physical nics for a zero-cost router. Not as fast as the real thing but they can do BGP, EIGRP, etc. Comes with a pix emulator too. Great for CCIE labs.
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