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      Simulating Network Architectures with GNS3

      “Problems in school are like problems in life, but without their consequences.”

      Olivier Reboul

      – Understanding the concept of simulation and its main theoretical aspects, as well as its capabilities and limitations.

      – Understanding the main network simulation tools.

      – Understanding how to use the GNS3 supervision tool and its features to model a network infrastructure based on specific needs.

2.1. Introduction

      As with the development of applications, system and network infrastructures, even simple ones, must be tested before they are put into production. This good practice is vital to avoid numerous errors during the production phase.

      Simulating and testing network infrastructure projects before deploying them concretely saves time and money, and ensures that they will function properly. A network simulator is a software program, free or paid, which is used to create virtual representations of IT networks.

2.2. Definition

      According to Wikipedia, network simulation is a technique by which a software program (simulator) models the behavior of a network, either using mathematical formulas to calculate the interaction between the parts of the network, or by recording and reproducing observations made of a real network.

      Simulation is a fictional representation of reality. It means to imitate a situation. The best-known example is probably that of flight simulators, which enable pilots to train without risk.

      In the context of networks, simulation refers to the reproduction of the architecture of a network without using any physical equipment. The interest of the simulation lies in its ability to test the features and operation of a network architecture at no cost and without hindering the normal operation of the production network.

      To do this, the simulation uses a software program that calculates (we will also use the term “modeling” to refer to this process) and thus predicts the events that will be led to happen, taking their features into account. There are many free and paid tools to conduct these simulations, including GNS3, Cisco Packet Tracer, Cisco VIRL, Marionnet, Boson network simulator, Eve, etc.

      A simulator can perform tasks to summarize the behavior of a network and its components, while an emulator can copy a network's behavior in order to stand in for it operationally.

      Network simulators enable an architect or network engineer to build and assess an experimental model of a network, including its topology and application flow, bearing in mind that the network itself is being implemented in the real world.

      In addition, network emulators enable architects, engineers and network engineers to precisely assess the reactivity, output and quality of an application before making changes or additions to a system.

Emulation is the ability to substitute something for the real object, while simulation has more to do with modeling the internal state of a target.

2.3. Introduction to GNS3

      GNS3 (Graphical Network Simulator-3) is a graphical network emulator that is free to use, open source and reliable. It can also be used to test all types of equipment, not only that of one manufacturer in particular. Thus, it can be used to test CISCO, Juniper, SOPHOS, Citrix and many others. It is also multiplatform and can be installed on Windows, Linux and Mac operating systems. It uses a framework of open-source programs, each of which can emulate specific platforms to execute software programs corresponding to a provider. It is used to test configurations that will be deployed in the future on real equipment.

      GNS3 is a network emulation tool similar to the Cisco tool PacketTracer, which is used to simulate a network infrastructure based on Cisco equipment and to configure equipment in order to reproduce the real infrastructure as closely as possible. GNS3 makes it possible to emulate networks more realistically, through both its ability to import real Cisco (or other) IOS into the routers emulated, and its ability to include in our virtual network real virtual hosts created with VirtualBox or other virtualization software. Thus, with GNS3, we can link the real to the virtual.

      It includes the following components:

      – Dynamips: emulates Cisco (or other) routers and switches and provides the corresponding environments or IOS;

      – Dynagen: represents the Dynamips control and management software layer;

      – Qemu: an emulator that enables GNS3 to execute Cisco ASA, PIX;

      – VirtualBox or VMware: used to create and run virtual machines linked to Windows and Linux operating systems.

      GNS3 is based on three concepts: simulation, emulation and virtualization. Since we have already discussed the concept of simulation, we will now introduce the other two concepts.

Emulation does not allow modeling, but actually identically reproduces the behavior of a software program and its hardware architecture. Virtualization uses the architecture of the host system, while emulation reproduces its software, which makes the virtualization perform more efficiently.

      2.3.1. Functionalities of GNS3

      – Simulates the behavior of two network interfaces connected via an Ethernet cable.

      – Emulates routers, switches and other computers or servers. The respective components are operated with real software. Consequently, each virtual router requires a real IOS image. The thousands of lines of code of the highest-performance routers will be executed exactly as they would be in the real world, thus yielding the same behaviors as in production.

      – GNS3 can be used to capture traffic between two virtual routers via Wireshark.