Layer 3 IOS image for GNS3 with Switching GNS3 is one of best and my favorite graphical network simulator use for simulating network topologies and network Labs virtually in your personal computer. So GNS3 provide you the economical way for testing your networking skills at home. For creating topologies in GNS3 you are required the Cisco real IOS images. You need to add these IOS image in GNS3 in order to simulate these IOS for Networking LABS. If you are new to GNS3 and not clear you can learn it from given link. 3725 Switch IOS image Currently GNS3 runs only the routers IOS images, these is not way for direct running of cisco 2900 series IOS images in GNS3.