This specific image is typically a Layer 3 (Router) image. If you need switching features (VLANs, Spanning Tree), you would look for a companion image with "L2" in the name.
It supports almost all the commands found on physical Cisco 15.x hardware.
The image is preferred over traditional Dynamips (.image) files because:
Cisco IOL/IOU images are strictly intended for Cisco employees and authorized partners. For a fully legal alternative, consider Cisco Modeling Labs (CML) , which provides authorized Cisco images for a yearly subscription.
This signifies it is a Mainline release, typically used for stability.
It is specifically compiled to run as a native process on Linux, making it much faster than emulating a MIPS processor. Setting it Up in GNS3
Cisco IOU requires a license file named iourc . This is a text file containing a license key linked to the hostname of your VM.