Monday, January 3, 2011

What CCNA Students Need To Know About GNS3

If you are studying for the CCNA exam, then you have to practice Cisco IOS commands in order to build your hands-on experience and pass the CCNA exam. To do so, you need a CCNA lab, which could be expensive for some people. Therefore, GNS3 consist a cheap option to build a virtual CCNA lab with the same capabilities as of the real CCNA lab except the switching commands. In this article, you learn what GNS3 is designed for, how to simulate network topologies, and how to install it and optimize it for better performances.

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GNS3 is the acronym of Graphic Network Simulator 3. It is a free tool commonly used for simulating Cisco network devices such as routers, firewalls, and Intrusion detection/Prevention systems. In addition, GNS3 allows you to emulate Juniper routers using QEMU. Therefore, It makes simulating complex network with Cisco equipments and Juniper equipments a simple task.

Unlike other network simulation tools (allowing you to emulate one version of each Cisco/Juniper device), GNS3 allows you to choose the Cisco/Juniper device’s version to be simulated. For example, your simulate a large amount of Cisco routers including the 2600 series, 7200 series, 3600 series, and so on. This is the same thing for Cisco PIX and ASA firewalls and Cisco IPS devices. GNS3 uses true images of Cisco devices’ operating systems to simulate Cisco/Juniper routers, Cisco firewalls, and Cisco IPS devices, which allows more realistic conditions of simulation.

GNS3 is equipped with an easy to use graphic user interface. It offers a workspace, which makes setting up complex network topologies to simulate an easy and funny task. To create network topologies, it is necessary to add to the IOS images of the Cisco routers and firewalls to be emulated to GNS3. To get an IOS image, try to buy a Cisco router or buy a Cisco IOS image from Cisco.

Once you build your network topology graphically, the configuration of the devices is done by the command line. Each equipment is accessible through telnet using your OS built-in Telnet software, SecureCRT, or Putty.

GNS3 id free software, which can be downloaded at Additionally, if you’re going to simulate simple networks with one to five devices, then a dual-core Pentium CPU is sufficient. However, if you’re going to simulate complex networks including VM machines and different devices (routers, firewalls, IPSs,), then your computer should a quad core CPU or an Intel Core i7 in order to get better performance.

By default, simulated devices take 100% of your CPU upon launching. Therefore, your next step after starting your devices on GNS3 is to adjust the IDLEPC parameter for each emulated device. To do this, you can either right click each device and choose “idle-pc”, or use the idlepc command in the command line interface of Dynamips. This way, you optimize your CPU performances and devices will work smoothly. However, this will not have any effect if your computer’s CPU speed is limited or you’re running a complex network topology with a lot of services that consume large amount of CPU cycles and memory space.

The CCNA exam is not an easy exam, and I suggest you to get CCNA lab and a CCNA lab workbook to pass you exam and build up your routing and switching skills.

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