One of the best Router Simulation tools around is not in fact a simulator, but an emulator. The difference is that while simulators attempt to approximately do the same tasks as Cisco's IOS running on a router, emulators actually run the real thing on your PC. You obtain real IOS and run it via the emulator so you get all the commands and functionality of a real router.

GNS3 is a free graphical network emulator that allows you to design complex network topologies and to launch simulations on them.To allow complete simulations, GNS3 is strongly linked with :

• Dynamips, an IOS emulator which allows users to run IOS binary images from Cisco Systems.
• Dynagen, a text-based front-end for Dynamips.
• Pemu, a PIX emulator.

GNS3 is a excellent complementary tool to real labs for administrators of Cisco networks or people wanting to pass their CCNA, CCNP, CCIP or CCIE certifications.

It can also be used to experiment features of Cisco IOS or to check configurations that need to be deployed later on real routers. This project is an open source product that may be used on multiple platforms, including Windows, Linux, and MacOS X.

Features

• Designing high quality complex network topologies.
• Emulating Cisco routers and PIX firewalls.
• Simulating simple Ethernet, ATM and Frame Relay switches.
• Load and save in Dynagen's INI-like format.
• Image export (JPEG, PNG, BMP and XPM).

You can download GNS3 here