What is an 'Unknown' device?
To start with, it is important to understand how OpManager categorizes the devices into servers, routers, switches etc. OpManager relies on industry standard protocols like SNMP, CLI, and WMI to 'identify' the devices. SNMP is the standard protocol across all device categories while CLI is specific to Unix-based servers, and WMI is specific to Windows environment.
A device is grouped under 'Unknown' devices view due to the following reasons:
Devices classified under 'Unknown' category
So, even if OpManager is able to ping and discover the device successfully, it does not know where to put the device and therefore classifies it under 'Unknown'! If you find any of your servers or other infrastructure device missing in the relevant category map, select the Maps --> Unknown category view and check if it is present here. You can move such devices to the relevant category, or rediscover them after enabling the SNMP/ CLI/WMI protocol and configuring the credentials. Refer to the troubleshooting guide for more details.