What is GNS3, the Most Preferred Simulator Program to Prepare for Cisco Exams?

GNS3; It is a software that provides an interface to emulation software such as Dynamips, VirtualBox, QEMU and enables the emulation and configuration of network systems with different devices (Cisco, Juniper, HP, Arista, Citrix, Brocade routers and switching devices) and different operating systems. A real Cisco operating system IOS can be run with Dynamips. With QEMU, Juniper operating system Junos, Cisco ASA, and IDS / IPS systems can be operated. In this way, it is possible to test different physical hardware with GNS3. With Virtualbox, it is possible to add computers that emulate different operating systems to the virtual network system. GNS3 can be installed on various operating systems.

If we compare GNS3 with another popular software used in education; Cisco Packet Tracer is a very widely used simulation software especially in the Cisco Network Academy Program. The biggest difference of GNS3 software from Cisco Packet Tracer software is that GNS3 is an emulator and Cisco Packet Tracer is a simulator. So while GNS3 runs the operating system used on a real router, Packet Tracer uses a software-defined virtual operating system. While this prevents us from using all configuration commands in the Packet Tracer software, all commands valid for IOS used in GNS3 can be used. Another important difference is that switching devices (switch) are not emulated in GNS3, while this is possible in packet tracer. In GNS3 software, switching devices can be used only as unmanageable switches. Although the switching devices available by default in the GNS3 software are unmanageable, this can be overcome by using routers as a switching device. With the module support provided by GNS3, there are a number of operations that need to be implemented to turn a router into a switching device. For example, this can be accomplished by adding the NM-16ESW module.

Building a Network with GNS3

GNS3 software needs more memory and a faster processor depending on the number of devices to be emulated. Considering today’s computers, the 4-core Intel Core i5 or equivalent processor is sufficient for satisfactory device emulation. The number of different images and different router models used in GNS3 is another factor that increases the amount of processor and ram usage.

For example; A computer with the following hardware is used to create a virtual network with 20 mid-range routers:

  • Intel Core i5
  • 16 GB RAM
  • 250 GB HDD

In the laboratory environment to be installed, Cisco 3725 IOS will be used and 256 MB DRAM will be assigned for each router. Since there are 26 devices that can be used by students in the lab environment, it means 6700 MB RAM for the created virtual 26 routers. Considering system needs and virtual machine requirements, it requires at least 8 GB of RAM and 16 GB of RAM for a healthy operation, but considering the benefits, GNS3 offers to us, these costs are very low.

Although the need for RAM and processor in GNS3 grows in parallel with the number of routers we add, the idle PC value should be used to limit/reduce excessive consumption. This value calculates the times of virtual routers emulated by dynamips, putting the virtual router to sleep. The virtual router that goes into the sleep state (idle) does not use a processor. When Idle-PC is not enabled in the system, it can increase the CPU usage to 100% and cause the machine on which it operates to not respond to requests. To avoid this situation, the correct idle PC value should be calculated.

Studies on a virtual router scenario with 20 routers gave the following results; From the moment the devices are started, the processor usage reaches 100% and the RAM usage reaches 6.4 GB. In the process of calculating the idle PC value, right-click each of the routers in the virtual network or select the Idle-PC option in one of the router groups using the same type of IOS. The system will calculate the values ​​and present us with the most appropriate value with the * symbol. By selecting these values, a reduction in processor and memory (ram) usage can be achieved. Sometime after the Idle-PC Value was assigned, the processor load dropped to 18% and RAM usage to 3.2 GB.

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