Well this is something that I ran into a few years back when a dummy connected his linksys router to our network and started serving up IP's. Well not only is this a pain in the ass to track down in a large campus but it can be prevented with this feature.
You can enable it at a global level via ip dhcp snooping or on a vlan via ip dhcp snooping vlan 50. you will then specify which ports belong to a trusted DHCP server:
Int gi 0/24
ip dhcp snooping trust
Verify Traffic:
show ip dhcp snooping binding.
More Info:
Catalyst 6500 Release 12.2SX Software Configuration Guide - DHCP Snooping [Cisco Catalyst 6500 Series Switches] - Cisco Systems
