There are some scenarios that are very challenging for administrators to simulate. Not only because the cabling is so complex or because there is no place for it - sometimes there is simply a lack of free equipment or the necessary time to simulate network building on test equipment. Who already has Exchange servers or Cisco routers in supply?

Virtualization in data centers and server rooms makes testing environments much easier to deploy. While all major hypervisor vendors are talking about virtual and software-defined networks (SDN), but two routers have to be properly balanced, the concepts of the visionaries are still a bit too far away. In addition, network and router simulators are usually not for those with a small budget. Fortunately, there is the Dynamips software, which makes it possible to virtualize practically any Cisco hardware on a standard computer. And if you like it a little easier, you can access the free GNS3 (GNS is a graphical front-end for Dynamips), a graphically enhanced complete package for network and server simulation for VirtualBox, VMware ESXi or Workstation / Fusion.

The core software, Dynamips, is a pedigree emulator. The program does not emulate the complete router, but only the underlying hardware. The Dynamips / GNS3 Administrator must make every effort to ensure the necessary operating system is used. Cisco customers and Cisco Networking Academy participants can download the images directly from the Cisco website, depending on the level of certification. Usually it's not a problem to find a supply of IOS images, but their handling in Dynamips is complicated. This is where GNS3 comes into play - on the homepage, simply visit the "Getting started" area, and the installation and configuration is explained. All you need is a computer with more than 4 GB of RAM, 1 GB of free disk space, a modern CPU with at least two cores, and AMD-V / RVI Series or Intel VT-X / EPT support. The more the better! GNS3 runs on Mac OS X, Windows and Linux.

If you want to be quick, search for the desired version on the Github website (for example, as a preconfigured virtual appliance for VMware Workstation). Just import the OVA file and start GNS3. Afterwards, the installation of the actual client software on your own PC is necessary. The GNS3 client software controls the virtual appliance referenced during the installation.

All in all, about 15 minutes have passed - now it is time to click "Add an IOS router using a real IOS image" and upload an image, (for example the "c7200-adventerprisek9-mz.124-9.T.bin", with 512 MB and the plug-in "PA-2FE-TX" as slot 1, next to the automatically created "C7200-IO-FE"). Then the emulator starts to determine the necessary idle value, otherwise the PC would always work with 100 percent CPU load and overheat. From now on, the IT professional can integrate a virtual router into the virtual network from the left menu via the router symbol. From the Context menu, the administrator connects directly to the console via PuTTY and can issue all Cisco commands to configure the device. How to configure basic settings and the imaginary cabling is self-explanatory. 

Particularly exciting is the possibility of integrating additional virtual machines into the simulated network: for example, to use a trial installation of a monitoring tool to test monitoring parameters. At first glance, however, the software only offers "VPCS" with a minimum operating system. If full-fledged VMs are available, the IT professional must select a VM under "Edit" in "Preferences" in the desired section. For example, select "VMware", and then select the "Allow GNS3 to use any configured VMware adapter". From now on, GNS3 is also able to integrate this VM into the intended network activity. Approximately 30 minutes later, a configuration is finished, in which the monitoring software looks at an emulated Cisco router via SNMP.

Helpful commands (Basic configuration): enable, show running-config, configure terminal, hostname {name}, enable secret {pwd}, no ip domain-lookup, interface fastethernet 0/0, ip address {IP / SUB}, no shutdown, snmp-server community public RO, snmp-server community private RW, snmp-server contact {Name}, snmp-server location {name}, write

Helpful Commands (Routing): router rip, no auto-summary, version 2, network {IP}

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