I would like to start my CCNP, however I would like to know which network sim I can use to do labs.
to get an IOS go to
you'll need to have a CCO login or register (if you have valid smartnet contract) in order to download.
I strongly agree with JON with dynamips. But if you want to make graphic network simulator I would highly recommend using GNS3, because is an excellent complementary tool to real labs for Cisco network engineers. It can also be used to experiment features of Cisco IOS or to check configurations that need to be deployed later on real routers.
caveat to caveat 1) above. i think if you use gns3 you can set a lower priority on the emulation so it doesn't slow the rest of your pc down quite as much.
Personally i would use dynamips -
This is software that actually runs the Cisco IOS on your PC/laptop and therefore provides full routing functionality. It can also emulate a frame-relay switch, an ethernet switch, an ATM switch.
2 caveats with the above -
1) You need a reasonably specced PC/laptop as dynamips is quite CPU and memory intensive
2) You need to have access to Cisco IOS images as dynamips actually loads these on your PC.