Use Ostinato to inject 802.1Q packets into the network for VLAN hopping (this video shows double tagging). Free software worth $200: https://goo.gl/613TUF
Ostinato – Network Traffic Generator Part 7. Easily create packets for testing in your GNS3 network.
Ostinato is a packet crafter, network traffic generator and analyzer with a friendly GUI. Also a powerful Python API for network test automation. Craft and send packets of several streams with different protocols at different rates. Think of it as “Wireshark in Reverse”.