Juniper recently started project called Juniper vLabs. It's a web-based platform that lets you try Juniper products in any time for free.
All that you need to use vLabs - is a juniper account which can be registered for free. System relies on blueprints - schemas/situations that you could try to play with in virtual environment without harm to your product environment. These blueprints available right now:
After you requested for blueprint deployment, system will tell you estimated time for it's preparation, and after this time you're free to do whatever you want with this devices. Also you should notice that your playtime is limited. I think time limit is based on type of blueprint. I get 3 hours of time for blueprint with single vMX router.
Here you can read little more details and continue with your first sandbox with virtual Juniper to explore and educate!
