Yersinia

Yersinia is a network tool designed to take advantage of some weakeness in
different Layer 2 protocols. It pretends to be a solid framework for analyzing
and testing the deployed networks and systems. Currently, the following network
protocols are implemented: Spanning Tree Protocol (STP), Cisco Discovery
Protocol (CDP), Dynamic Trunking Protocol (DTP), Dynamic Host Configuration
Protocol (DHCP), Hot Standby Router Protocol (HSRP), IEEE 802.1q, Inter-Switch
Link Protocol (ISL), VLAN Trunking Protocol (VTP).

The best Layer 2 kit there is.

Get Yersinia Here - http://yersinia.sourceforge.net/