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: PIX and ASA firewalls, Ethernet switch, ATM bridge, ATM switch, Frame Relay switch, Cloud, Qemu and VirtualBox guests etc.
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.
The middle section contains two panes. The top pane is your work area where a topology may be graphically built. The bottom pane, called the Console, shows Dynagen at work. Dynagen, as you recall, is the text-based front end to Dynamips, the core emulator being used. Learning how to use Dynagen is like learning how to use DOS the first time, so we will not get into that here. However, we will use a very few simple but useful commands in the Dynagen pane.
Configuring a router
Click on a router icon under Nodes Types corresponding to the IOS platform you are using. In our example, we are using a 3640 platform. You must use a platform for which you defined an IOS. Drag an appropriate router node type over to the workplace pane in the middle and let go. We now have a router ready to configure. Right-click the router and choose Configure.
Click on R1 and then the Slots tab. Click the drop-down arrow next to slot0 and choose an adapter that includes FE in its description. This will add a FastEthernet adapter to the router. Next, click the drop-down arrow next to slot1 and choose PA-4T+, PA-4T, or NM-4T (if you do not have these exact adapters, just choose something close.). This will add four serial interfaces to the router. Click OK.
Right-click the router and choose Start. Right-click the router again and choose Console. A Putty console opens up.
You may need to press Enter once initially in the Console window. After a few seconds, your virtual router should have started.
Apply an idlepc value
Now it’s time to choose a idlepc value. Be sure you can see the prompt of your router in Putty window. Right-click R1 and choose Idle PC. GNS3 will spend a moment calculating an Idle PC value before presenting the screen to the right. If you click the drop-down arrow, you see a list of possible idlepc values. Potentially better idlepc values are marked with an asterisk. Choose one of the values with an asterick (in our example, we will choose number 1) and click OK.
If you choose IOS images and hypervisors on the Edit menu, and double-click on the image under the IOS Images tab, you’ll see the new idlepc value displayed under Settings.
You may repeat this process to find the value that reduces CPU usage the most, but this time use the Apply button so you can instantly see the effect of a idlepc value while observing the CPU usage. To observe CPU usage in Windows, press Ctrl+ALT+DEL and choose Task Manager. Click on the Performance tab to view CPU usage. In Ubuntu, choose System Monitor under Administration on the System menu. Click the Resources tab.
You will observe that without an idlepc value, CPU usage will be at or near 100%, but with an idlepc value, CPU usage will drop to a very low value.
Enjoy using your router!
You may now return to your Putty window to use your router. You are actually running the Cisco IOS that you chose. All commands supported by the IOS are available.
Remember earlier we chose a FastEthernet adapter and a four-port serial adapter. If you issue the show ip interface brief command as shown, you’ll see the designations for these four ports on the router. In our example, they are fa0/0, s1/0, s1/1, s1/2, and s1/3 (in abbreviated form).
Ok but a single device is not so useful to create a virtual network! Let’s build a more complex topology.
