GNS3 Initial Configuration

GNS3 Initial Configuration Tutorial

With the introduction of dynamips as a Cisco IOS emulation tool, it is now possible to emulate almost any Cisco IOS image in a simulated hardware environment. Of course the main problem with dynamips was that it was a CLI tool that required a lot of parameter knowledge. To make the configuration easier, the dynagen tool was developed to provide an INI file-like configuration interface. For most of the old school engineers out there this was familiar, and easy to figure out and configure. GNS3 was developed to take the ease of the configuration to a whole new level; this included a GUI-style interface that most people familiar with Windows based systems understood (almost anyone with a computer). GNS3 added additional abilities to the dynamips platform including support for Cisco PIX and ASA as well as Junipers – JunOS. This article is a second in a series of GNS3 configuration articles that aims to make the simulation of Cisco IOS environments easier to understand and get those engineers out there looking to become Cisco IOS proficient a stepping stone.

Before this article goes any further, we should point out that the only way to have either of these products emulate a real IOS environment is to already have access to an existing IOS image. Dynamips, dynagen and GNS3 don’t include these images (this is because the legality is in question). Before anyone can use these products, they must obtain one of these images for the platform being emulated. There are ways to get these legally (purchase a Cisco SMARTnet contract or have an existing Cisco IOS device) and not so legally (look online); this article will not condone or approve the method used in obtaining this image. For the purposes of this article, an image for a Cisco 7206 platform (7200) will be used.

GNS3 Initial Configuration

The main parts of the initial configuration of GNS3 to ready it for Cisco IOS emulation is the configuration of Cisco IOS images into GNS3 and the configuration of any non-default preferences. The following group of figures will show the different steps required to set up Cisco IOS images and walk through the different preference options that can be configured when emulating Cisco IOS.

Figure 1 shows the main GNS3 screen.

Figure 1 - Main GNS3 Screen

Figure 1 – Main GNS3 Screen

Figure 2 shows the menu item to be selected to configure the Cisco IOS images.

Figure 2 - IOS Image Menu Path

Figure 2 – IOS Image Menu Path

Once the IOS images and hypervisors menu option is selected, the screen shown in Figure 3 will be displayed. On this screen the path to the IOS image can be selected by clicking the browse button as shown.

Figure 3 - IOS Image Selection

Figure 3 – IOS Image Selection

Once the browse button is selected the file selection window shown in Figure 4 will appear; browse to the location of the image and select it and press the open button.

Figure 4 - File Selection Window

Figure 4 – File Selection Window

Typically, the Platform will auto populate if the IOS filename remains within Cisco’s naming convention and the model options available will populate the Model dropdown box. As shown in Figure 5, the platform of the selected IOS image is c7200 and the only model supported is a 7200. If for some reason the auto-population did not work, select the appropriate platform and model.

Figure 5 - Platform and Model Population

Figure 5 – Platform and Model Population

One thing to note is that the IDLE PC value is important but will be populated when the first lab is run with this IOS image and the IDLE PC process is completed; just leave it blank for now. Once the options are correct select the save button, once this is done a screen like that shown in Figure 6 will appear. Once the configuration is saved select the Close button.

Figure 6 - Cisco IOS Image Configuration – Save

Figure 6 – Cisco IOS Image Configuration – Save

The second part of this article will display the screens used to configure the different preferences available for GNS3, generally speaking the default settings should work for most people. Figure 7 shows the menu item to be selected to configure GNS3 preferences.

Figure 7 - GNS3 Preferences

Figure 7 – GNS3 Preferences

Figure 8 shows the main general preferences screen; there are a couple of different options here that someone may want to alter including the project and OS image directories.

Figure 8 - General Preference Screen

Figure 8 – General Preference Screen

The terminal settings screen shown in Figure 9 can be used to select the preferred terminal application. Putty is generally included with the GNS3 installation, but SecureCRT, Telnet and Teraterm are supported as well.

Figure 9 - Terminal Settings

Figure 9 – Terminal Settings

The Dynamips configuration options can be altered if required, however, the default options will work for almost all individuals.

Figure 10 - Dynamips Options

Figure 10 – Dynamips Options

The one option that may need to be altered is the amount of memory that is allowed to be used per hypervisor (each session of dynamips); value will be needed because this will depend on the Cisco IOS image memory configuration. For beginning configurations, the default configuration will work fine. If this memory usage limit does need to be configured the screen shown in Figure 11 should be used.

Figure 11 - Dynamips Hypervisor Configuration

Figure 11 – Dynamips Hypervisor Configuration


Once the Cisco IOS images have each been configured (configure each image that will be used in the emulation), GNS3 is then ready to be used. The next article in this series will cover how to setup a single Cisco IOS device within GNS3.  Hopefully the contents of this article will enable the reader to get to a point where this is easily completed.

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