09-08-2012, 11:54 AM
Graphical Network Simulator
GNS3-0.5-tutorial.pdf (Size: 6.65 MB / Downloads: 248)
GNS3
GNS3 is a Graphical Network Simulator that allows emulation of complex networks. You may be familiar with VMWare or Virtual PC that are used to emulate various operating systems in a virtual environment. These programs allow you to run operating systems such as Windows XP Professional or Ubuntu Linux in a virtual environment on your computer. GNS3 allows the same type of emulation using Cisco Internetwork Operating Systems. It allows you to run a Cisco IOS in a virtual environment on your computer. GNS3 is a graphical front end to a product called Dynagen. Dynamips is the core program that allows IOS emulation. Dynagen runs on top of Dynamips to create a more user friendly, text-based environment. A user may create network topologies using simple Windows ini-type files with Dynagen running on top of Dynamips. GNS3 takes this a step further by providing a graphical environment.
Creating the Simplest Topology
We describe how to build a more complex topology later, but for now, let’s just learn how to place one router on the desktop, start it, and console into it. We will then learn how to find an idlepc value for the IOS we are using. This is a very important step. When an IOS is running, it will consume up to 100% of your CPU time. This will cause your computer to become very sluggish and will prevent building more complex topologies. However, if we use an idlepc value, we can reduce CPU usage dramatically. It puts the IOS into a sleep state when it is not in active use and wakes it up only when it is necessary. A more technical explanation will be given later.
The GNS3 window is divided into four panes by default. The left-most pane lists the types of nodes available. You will see router icons for the various platforms, a PIX firewall, Ethernet switch, ATM bridge, ATM switch, Frame Relay switch, and Cloud. Other node types may be added as explained later.
The right-most pane will provide a topology summary that will be better understood when we built more complex topologies. For now, just know that the pane exists.
Building More Complex Topologies
Although a single router is useful to get familiar with commands, it would be nice to build more complex topologies.
With GNS3 very complex topologies may be built. Just as performance when running virtual machines within VMware or
Virtual PC depends on your computer resources, running many router instances will affect the performance of your
computer. The faster your CPU and the more RAM that you have, the better. However, idlepc does a lot to help with
CPU usage. Ghostios and sparsemem are two additional utilities that will help with RAM usage. Ghostios is enabled by
default in GNS3. Sparsemem must be enabled, since it is off by default. These will be discussed in detail in the section on
Memory Usage. As an example, with 2 Gigabytes of RAM and a 2.5 GHz processor, you should have no problems running
a lab with good performance with a half dozen routers and several workstations.
Let’s build the topology shown with three routers. Start GNS3. Drag
three routers running an IOS you have configured into the
workspace. Right-click each router and choose Configure. Under the
Slots tab, include a FastEthernet adapter and PA-4T serial adapter.