I suspect everyone reading my blog uses GNS3. If you don’t, go check it out. From their website:
GNS3 is an open source software that simulate complex networks while being as close as possible to the way real networks perform. All of this without having dedicated network hardware such as routers and switches.
They are working towards 1.0 and are getting close. They are currently offering early access a year before the public release if you support the GNS3 1.0 Early Access CrowdFunding campaign.
Unlike products such as Juniper’s Junosphere and Cisco’s VIRL/CML, GNS3 is multi-vendor and multi-platform. This is key for learning and for validating real-world networks, since very few networks don’t have at least some mix of vendors.
Most of us use GNS3 and find it valuable for labbing, whether for certification study, validating configuration changes at the office, or just to test network behavior. It’s worthwhile to give these guys a few bucks to improve and enhance this free product that has been invaluable to the network community. I did and recommend you consider it, yourself.