This is rather clever….
What is VoIP Hopper?
VoIP Hopper is a GPLv3 licensed security tool, written in C, that
rapidly runs a VLAN Hop into the Voice VLAN on specific Ethernet
switches. VoIP Hopper does this by mimicking the behavior of an IP
Phone. It first dissects either IEEE 802.3 or Ethernet II for Cisco
Discovery Protocol (CDP) packets. If CDP is enabled on the switch
port and the Voice VLAN feature is enabled, it will determine the
Voice VLAN ID (VVID). This will allow the tool to create a new
Ethernet interface on the PC that tags the 802.1q VLAN header in the
Ethernet packet. After VoIP Hopper has created the new Ethernet
device, it will send a DHCP client request.
You can get it from http://voiphopper.sourceforge.net/