GNS3

GNS3 is a excellent complementary tool to real labs for administrators

of Cisco networks or people wanting to pass their CCNA, CCNP, CCIP or CCIE

certifications.

It can also be used to experiment features of Cisco IOS or to check

configurations that need to be deployed later on real routers.

Important notice: users must provide their own Cisco IOS to use GNS3.