SNMP autodiscovery

Hi forum -
I am implementing SNMP code to autodiscover Digi Passport devices. My Java code is written with the snmp4j library, using an SNMP get broadcast on the public MIB variable sysObjectID . My code in general works, because I see that several other non-Digi vendors respond correctly. However the Digi devices do not respond. Note that if I do a direct SNMP get sysObjectID on a specific Digi IP address, I do get a response. Anyone have an idea how to resolve this ?