Here i will introduce you with Cisco IOU which will help you for your CCIE R&S Exam.
Over the internet you will get different version of Cisco IOU but all IOU are same as the original one , The difference is the topology.
You can modify the topology by editing the NETMAP file . NETMAP file is the same file that we use in GNS. ( Please see How to Create NETMAP in other post )
In IOU you can get Linux Base IOS, Below are the file name of Cisco IOS
In advance enterprise IOS you will get all feature that you get in Cisco 7200 series router with K9 image ot Adv IP Base IOS.
Using GNS is good but when you what to run 20 or 30 router then it will create problem.
GNS is a emulator that emulate the hardware with original IOS so it require more CPU.
If you want to run 20+ router in GNS then it will kill your CPU. So now you can go with Cisco IOU .
In Cisco IOU you can 50+ routers or may more.( Req RAM not CPU )
In up coming post i will let you know :
1] How to run Cisco IOU
2] How to Save Configuration
3] And many more ...