GNS3 is a free and open source bit of kit for professional network engineers. GNS3 enables a network engineer to emulate, configure, test and troubleshoot virtual and real networks. GNS3 can run on anything from a raspberry pi to a fully featured cloud based compute instance, the cloud instance needs to have kernel-based virtual machine (KVM) support.
At the time of writing GNS3 is used in large companies worldwide. Some notable mentions:
Pros of GNS3:
- It’s free!
- It’s open source
- No monthly or yearly license fees
- No limitation on number of devices supported such as with some other solutions. (you can just google around to find some examples, I am not going to name them here).
- It supports multi vendor environments, meaning if you actually wanted to know if eigrp works with non cisco equipment you can do it with gns3.
- There are free, pre-configured appliances and appliance files for almost every vendor, operating system. If it has an ethernet port it is probably in market place. You can make your own appliances as well.
- Highly supportive community.
- Makes a lot less noise than an actual stack of networking equipment booting up. The equipment may even boot up faster*.
- Wireshark network protocol analyser integration with no fuss just right click a cable and select start capture.
Cons of GNS3:
- You need to supply your own images for Cisco. (can be downloaded from cisco.com or purchased for a year with a VIRL licence. Additionally, a copy from a physical device.)
- Not the easiest in the world to setup if you are transitioning to IT. (The community will help you, without laughing at you)
- Depending on the complexity of the network and the scale, GNS3 can be affected by the running environment (your pc).
An example of where GNS3 was used in a small group project: https://williedeklerk.com/2024/09/26/practical-application-of-ospf-authentication/