As a network technician, it is important to be able to continually practice your job, learn new things, and work to optimize the network when it’s in use. In a perfect world we would have a fully functioning lab environment where we can duplicate the tactical network, experiment, and roll out configuration changes in a controlled environment to see what effect they will have on the real thing. Unfortunately, that is not the case. Many Brigades have little or no spare equipment that can be put into use. Using the real thing when it is not in system is always an option, but often very hard to do due to the numerous owners and training calendars involved.
With using the real equipment not being a serious option, we have to look for other possibilities. There are a large number of network simulators available, but they are only a partial solution. While simulations offer a fair bit of functionality, they do not support many of the commands that are commonly found across the WIN-T network. So where does that leave us?
GNS3 (Graphical Network Simulator) is a fully functioning router emulator. Unlike a simulator which simply provides a similar experience to the real thing and attempts to react in the same way, an emulator actually loads up the real Cisco IOS and runs it. This means that nearly anything that the router can do, the emulator can also do because it is actually operating off of the IOS.
So if this is the case, why doesn’t everyone just stop buying real routers for their labs and use GNS3 instead? Well a few reasons. First, running a “router” on your desktop computer requires a fair bit of resources and isn’t capable of handling nearly the same amount of traffic that a real router can. While this isn’t a huge deal for a simple lab environment it is definitely not a replacement for a production network.
Second, while GNS3 is capable of making a fully functioning router, it does have its limitations. Currently there are only a handful of router platforms that it can emulate (currently Cisco 1700, 2600, 3600, 3700, and 7200 series routers). Additionally, GNS3 does offer some basic switch functions, it is not even close to being functional (so if you need anything more than basic layer 2 switching, you’ll need the real thing). Finally, GSN3 is capable of emulating a Cisco PIX or ASA security appliance including the use of ASDM.
One final thing to remember is that while GNS3 is able to emulate all of this, it does require a functioning copy of the IOS which is not included. If you have access to this, then you are good to go. If you do not, there are a variety of places online where you can get copies. StartCCNA can help point you in the right direction. Also currently GNS3 is only able to operate IOS 12.4 or below.
I will not walk you through how to use GNS3 to setup a basic routing network (there are plenty of tutorials you can find with just a simple Google search) but later on I will try and put together a tutorial on getting the ASA to work because I had problems with that.
GNS3 can be found at http://www.gns3.net/. Reading the site’s news release it appears that they are working on the next version for release in late 2014 which will include a large number of improvements (including additional switch functionality).