VIRL does not emulate any specific hardware platform. VIRL runs special builds of Cisco's operating systems that are designed to run as virtual machines. Cisco has taken IOS, IOS-XR, and our other network operating systems, cut them off at the hardware abstraction layer, and ported them over so that they can run as a VM in a network simulation. For this reason, features of the Cisco OS implemented entirely in software generally work exactly as they would in a device running the same version of the corresponding operating system (IOS, IOS-XR, etc.). This includes most of the control and management plane features of the OS. You should see the same behavior and the same bugs found on the associated version. Cisco OS features that are executed on the hardware (ASIC) may not be supported unless Cisco has added a software-based implementation of the feature. The layer-2 switch image (IOSvL2) in VIRL, is an example where features implemented on an ASIC have been implemented in software. The router virtual machines in VIRL are rate-limited, with no sense of a synchronized CPU clock across them. This is virtualization, not emulation. VIRL is very good for troubleshooting policy and configuration issues of supported features, but it's not generally useful for testing throughput or timing.
Configurations taken directly from a physical Cisco device may not function as expected when applied to a Cisco node in a VIRL topology. Depending on feature sets configured in your production environment, you may be able to closely simulate your production topology and configuration. VIRL also provides tools for traffic generation (iPerf, Ostinato), packet capture, and Live Visualization to help you test and understand how traffic flows through your simulated network.
For a complete list of supported features bundled in the included Cisco virtual machine images, visit https://learningnetworkstore.cisco.com/virlfaq/features or VIRL's Documentation Index on The Cisco Learning Network.
I hope that clarifies what VIRL is and what it is not, and helps you decide whether VIRL is suitable for your networking needs.
Cisco Learning Network