Basic Configuration for GNS3 Labs

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Now that the process of configuring the device has been covered in the previous article, GNS3 Initial Configuration, this article will take a look at how to setup GNS3 with two devices and then connect them together with a serial cable (virtually of course). This article will also go over the process of establishing the IDLE PC value.

GNS3 Basic Lab Configuration

For those just learning GNS3 it can be take a little time to figure out exactly how it works. Since many people just learning GNS3 are also just learning networking, it can be even harder. GNS3 was designed to take some of the complexity out of emulating Cisco IOS environments (among others). The other tools that are available (dynamips/dynagen) are still used by GNS3, but are just configured and operated in the background. The configuration of labs within GNS3 requires some basic level GUI OS proficiency which most people already have.

To begin with, it is best to first have an idea of what the topology is going to look like. For this article the topology is going to connect two 7206 routers together with a single serial link. The first step to set this up is to drag the two 7200s from the nodes types pane to the map pane. This process is shown in Figures 1-4 below:

Select the 7200 Series Router

Figure 1 - Select the 7200 Series Router

Drag the First Router to the Map pane

Figure 2 - Drag the First Router to the Map pane

Select the 7200 Series Router Again

Figure 3 - Select the 7200 Series Router Again

Drag the Second Router to the Map pane

Figure 4 - Drag the Second Router to the Map pane

Now that the two routers have been placed on the map pane, the two need to be connected together with a serial link. The Add a Link tool is in the top navigation bar and resembles a cable connector, once it is clicked the different types of link available are shown for selection. In this case a serial link is going to be used to connect the two devices. The selection of a Serial link is shown in Figure 5.

Add a Link Tool - Selecting Serial Link Type

Figure 5 - Add a Link Tool - Selecting Serial Link Type

Once the link type has been selected, connecting together devices is as simple as clicking the first device and then clicking the second device; this process is shown in Figure 6-8. Once the link has been successfully created, the map pane will reflect the link between them with red dots, these dots indicate that the interface is not enabled (in this case the router has not been started yet.).

Select the First Device

Figure 6 - Select the First Device

Select the Second Device

Figure 7 - Select the Second Device

A Completed Serial Link

Figure 8 - A Completed Serial Link

Once all of the Serial links have been completed, the Add a Link tool needs to be disabled; this is done by clicking on the Add a Link tool again. When it is active, the thumbnail will change to a red X. Figure 9 shows the thumbnail that will need to be selected.

Disabling the Add a Link Tool

Figure 9 - Disabling the Add a Link Tool

Once you have disabled the Add a Link tool, the lab and the two devices is complete. These following parts cover the steps required to configure the IDLE PC value; the IDLE PC value is important as it enables GNS3/dynamips to more efficiently use the host processor. Without it, the process utilization of the host machine will often be at 100%. To begin, a single device needs to be started and allowed to completely boot up. Figures 10-12 show the process of starting R1 and displaying the console. Watch the console until the device has been completely booted up and no additional messages are being displayed.

Starting R1

Figure 10 - Starting R1

Displaying R1's Console

Figure 11 - Displaying R1's Console

Watch the Console

Figure 12 - Watch the Console

Once the device has booted up and is idle, the IDLE PC value can be calculated. Figures 13 and 14 show the process of calculating the IDLE PC value and selecting the best value. The best values that GNS3 believes will provide the least processor utilization are shown with a ‘*', so select the one that provides the least processor utilization.

Calculating the IDLE PC Value

Figure 13 - Calculating the IDLE PC Value

Select a Value with a '*'

Figure 14 - Select a Value with a '*'


The process of calculating the IDLE PC only needs to be done once for each router model type, once it is calculated it will automatically be added to the configuration of all other similar models. Hopefully the content of this article enables the reader to set up a simple lab and become familiar with the process of using GNS3 and will help in future studies.

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Sean Wilkins

Sean Wilkins is an accomplished networking consultant who has been in the IT field for more than 20 years, working with several large enterprises. He is a writer for infoDispersion and his educational accomplishments include: a Master’s of Science in Information Technology with a focus in Network Architecture and Design, and a Master’s of Science in Organizational Management. Sean holds certifications with Cisco (CCNP/CCDP), Microsoft (MCSE) and CompTIA (A+ and Network+).