CDP via SNMP (gets CDP MIB from devices) is one way. It also uses routing tables and if you want ping sweeps (see attached screen shot). So make sure you have the Read-Only community string defined in the discovery settings.
Adding to Rob's comments, I have a whitepaper on how CiscoWorks Discovery works at https://supportforums.cisco.com/docs/DOC-9005 . That should help you with some of your Discovery decisions.
