The best free alternative to GNS3 is Cisco Packet Tracer. If that doesn't suit you, our users have ranked six alternatives to GNS3 and four of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to GNS3 are Dynamips (Free, Open Source), Eve-NG (Freemium, Open Source) and Free SNMP Agent Simulator (Freemium).
Packet Tracer is an essential learning tool used in these Cisco Networking Academy courses: IT Essentials, CCNA Routing and Switching, CCNA Security, Intro to Internet of Things (IoT), and Mobility Fundamentals.
The new EVE-NG platform is ready for todays requirements. It allows enterprises, e-learning providers/centres, individuals and group collaborators to create virtual proof of concepts, solutions and training environments.