Emulated Ethernet Switch Functionality: GNS3 Lab Configuration

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Now that the basics have been covered for the set up of GNS3 in the previous articles it is time to take a look at how to use its capabilities to emulate a real environment. This article will take a look at the configuration of GNS3 using a emulated Ethernet switch.

GNS3 Ethernet Switch Capabilities

The Ethernet switch that will be used by GNS3 is part of the Dynagen package that is used by GNS3 to emulate routers. While not the equivalent to a Cisco Catalyst switch it can be configured in a number of different ways that allow support for most common scenarios. Some of these capabilities include the ability to perform Ethernet Switching (if that wasn't obvious), as well as support for access VLANS, 802.1Q tagged VLANs and QinQ (Stacked) VLANs. Other than direct management, these are the features that make up a basic managed switch.

GNS Lab Configuration

The configuration of a lab that utilizes the emulated Ethernet switch functionality is rather simple once the configuration of the individual devices has been completed. The following figures will walk through the setup of a basic GNS3 lab using three routers and a single Ethernet switch with default settings that include all devices on the same VLAN (1).

Figure 1 shows the beginning configuration of the lab by dragging each of the devices from the Nodes Types pane to the Map pane.

Beginning Map Configuration

Figure 1 - Beginning Map Configuration

For this example, three different routers will be used, each of which have been dragged to the Map pane and configured with 2 Fast Ethernet interfaces.

Completed Router Map Configuration

Figure 2 - Completed Router Map Configuration

The next figure shows the configuration of a Ethernet switch that has been dragged over to the Map pane. For this example the default configuration will be used but it is important to know where the different options could be configured if needed.

Configuring GNS3

Figure 3 - Configuring a GNS3/Dynagen Ethernet Switch

To configure these options Right-Click the switch and select the Configure menu option. Once this has been done the screen shown in Figure 4 will be displayed. From here you will select the Ethernet switch name from the left pane.

Ethernet Switch

Figure 4 - Ethernet Switch Node Configuration Screen

From the next screen shown in Figure 5 the user can configure the Ethernet switch based on the requirements of the specific lab. By default, there are 8 different switchports that will be available that are all configured as access VLAN ports on VLAN 1. From this screen any specific switchport configuration could be completed, setting up additional links between Map pane devices.

Switchport Configuration

Figure 5 - Ethernet Switch Switchport Configuration

Now that both the routers and the switch have been configured it is time to configure the links between the devices and the Ethernet switch. To do this select the Add a Link button shown in Figure 6.

Add a Link

Figure 6 - Add a Link

The next few figures show the configuration of the links between the routers and the Ethernet switch (Note: These figures utilize GNS3's manual link mode available under Preferences).

Connecting R1

Figure 7 - Connecting R1

To SW2

Figure 8 - To SW2

Connecting R2

Figure 9 - Connecting R2


Figure 10 - To SW2

Connecting R3

Figure 11 - Connecting R3

To SW2 Again

Figure 12 - To SW2

Once all of the links have been configured, exit the Add a Link mode as shown in Figure 13.

Exit Add a Link Mode

Figure 13 - Exit Add a Link Mode

At this point the lab is ready to be started, which is shown in Figure 14. Once all of the devices are up and running each of the routers can be configured the same as they would if there were three physical routers connected to a physical Ethernet switch.

Completed and Running Emulation

Figure 14 - Completed and Running Emulation


All things being equal, the configuration of GNS3 is rather simple once the basics are covered and the student has a couple different lab configurations under their belt. The Ethernet switch functionality of GNS3/Dynagen is very simple and straight forward but it is limited and this should be considered when planning the lab topology. Hopefully the content of this article will enable the reader/student the ability to get GNS3 up and running and configured in the scenario being tested quickly and without much delay. This way time can be be spent on learning the networking system and not GNS3.

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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+).