So you want to get Cisco certified, but you don’t have a networking lab of your own. Maybe you aren’t sure whether it’s worth the investment yet, especially if you’re going for an associate-level certification. Maybe you don’t have the space for a bunch of Cisco equipment, or the cash. Whatever the reason, you have a much, much higher chance of passing the certification exams if you get some hands-on practice, and for the professional- and expert-level exams, practice is essential.
Now, there are a lot of practice simulators out there. Boson publishes one, and a quick Google search yields quite a few more results, including NetworkSims, which I’d actually like to try at some point. Of course, the majority of these you have to pay for, and most of them include predefined labs geared toward specific exams or certifications and don’t let you experiment on your own. Most are also Windows-only. Then there are network emulators, which use the actual IOS images. One of the best free ones in this category is gns3, which gives you a lab that is completely open-ended and runs on virtually any platform since you can compile from source yourself.
Gns3 uses Dynamips, Dynagen, and Pemu to emulate the Cisco IOS® and PIX firewalls, and either WinPcap (Windows) or libpcap (*nix) to capture network traffic (on your simulated network only!) for analysis. It can integrate with Wireshark (Ethereal) for packet analysis similar to how you would use it on a physical network, and when combined with VMware you can even simulate end users and their network traffic! Theoretically you could use something like VirtualBox or other virtualization software to do the same thing. [Guide (pdf)]
Of course, all this awesome capability does come with a price: you have to find your own Cisco IOS® images. That requires you have (or someone you know has) access to a Cisco corporate account, such as through a company or university. Once you have these images, you can extract them for faster loading when you launch gns3 or leave them as-is.
The gns3 website has a lot of good user tutorials on how to get started and how to tweak your setup. The gns3 wiki has practice labs and other information contributed by the community. I used gns3 to help me practice for the CCNA® exams, and continue to use it while pursuing the CCNP®. I would consider it a very valuable (and best of all, free!) tool to use whether you’re studying for a Cisco® certification or testing a network setup before putting it into production.
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