Use Ostinato to inject Spanning Tree BPDUs into your GNS3 network to make your PC become the root of the Spanning Tree.. Free software worth $200: https://goo.gl/613TUF
Ostinato – Network Traffic Generator Part 8. 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”.