When I try to descover devices in DFM the majority of devices go into the Questioned state as an SNMP Timeout, a few are known and some are stuck in the learning state at 40%. I have tried adjusting the SNMP timeout settings and deleting all Devices and reimporting them again into DFM with no luck!
Ah, I bet I know the problem. What we have seen is either customers copy the snmp-server engineID from device to device (which is wrong), or they are bitten by a bug where multiple devices come standard with the same engineID. The latter affects mainly Cat4500 switches. If you post the sniffer trace, I can identify which is the problem.
The solution/workaround for both is to make sure the engineID values on each device are unique, then remove the devices from DFM, and re-add them.