Cisco Router Emulator supported by GNS3 Community

Dynamips emulates Cisco 7200/3600/3725/3745/2691 Routers on a traditional PC.
You can use dynamips to create labs. It uses IOS Images (which are not part
of this package). Of course, this emulator cannot replace a real router. It is
simply a complementary tool to real labs for administrators of Cisco networks
or people wanting to pass their CCNA/CCNP/CCIE exams.