By Tom Weekley

There is a program available called GNS3 (Graphical Network Simulator) that allows a user to experiment with features of the Cisco IOS or check configurations before deploying them on real routers.  It is similar to Packet Tracer except it provides a hardware platform (like VMWare) and runs software (the Cisco IOS) on the virtual hardware.  This gives you the full functionality of the Cisco IOS instead of trying to emulate the expected output.  The program is open source and free for download from http://www.gns3.net for Windows and Linux.  I’ve gotten it to work under Fedora 10 and have seen snapshots of it running on Ubuntu 8.04.

I’ve had the opportunity to experiment with the software and have found it to be very useful.  The software will max out the CPU and eat up available memory with just a couple routers running.  There are tweaks in the configuration menus that allow you to manage your system resources.  A setting called ‘IdlePC’ lets you set a value for the CPU to use when running a certain hardware platform.  This lets the CPU share cycles with other hardware platforms and software programs on your desktop.  A ‘ghost ios’ setting allows you keep one copy of the IOS in memory and share it among multiple routers using the same hardware platform.  Documentation on how to configure these settings are available in a pdf tutorial on the website.  The program ran okay on my 939 desktop with 4 gigs of memory, but other programs were somewhat slow to respond to input.  It ran better on my dual-core laptop with 2 gigs.  I would recommend keeping the amount of running routers around 6 to 8 unless you have a beefier system (which I’m sure most of you gamers do).

The program only provides hardware platforms for Cisco routers and PIXs.  There is an 8-port fastethernet switch available, but it is unmanaged.  One hardware platform I noticed was missing was the 1800 series, but there are other common platforms available: 1700, 2600, 3600, and a 7200!  To console to the routers, an integrated program called Dynamips provides telnet ports for you connect to.  Simply right-click a router, select Console, and the Windows telnet program will start.  You can configure the software to use Putty telnet instead of Windows telnet if you wish.  I would recommend downloading WinTabber which allows you to consolidate your telnet windows into tabs in one window (similar to Konsole).

The one sweet thing about this program is you can connect your configured topology to a live network.  Download Virtual PC Simulator from http://wiki.freecode.com.cn/doku.php?id=wiki:vpcs and follow the configuration documentation.  Essentially, you’ll configure a computer icon as a cloud, map the cloud to your NIC, and when you connect a router’s interface to the computer icon, you now have network connectivity to your topology.  I tested this with a Windows Server on my network issuing certificates to routers for IPsec.  It worked fairly well.

It is also possible to perform a packet capture on an interface between two devices.  Simply right-click a link and select capture.  This will open Wireshark and display the captured output.  You will have to click refresh in Wireshark because the program actually writes to a capture file instead of Wireshark directly.

It is possible to save and restore topologies but it seems to be a little buggy on some platforms.  Another nice perk is when you save, the program will export your running and saved configured to a directory.  This seems to spit out errors if you have a router in the topology that you haven’t saved the running-config to nvram yet.

The program will crash at different points.  Most of mine seemed to happen when I used a compressed image on some of the routers or if I had an ethernet switch in the topology.  One minor bug I noticed was between two routers with a serial connection.  If I ran ‘show controllers serial’ on the routers, both routers stated that they had the DCE end connected.  I set a clock rate on each end and it worked fine.

Overall, GNS3 is a nice program.  It’s only on version 0.6 so expect to see bugs here and there.  The advantages and features available with this program make a worthwhile download and a must-have if you are learning Cisco or networking.  All you need is the GNS3 installation files, a couple third-party programs mentioned above for full functionality, and an IOS image for a given router platform.