With IPHost Network Monitor you can run simple snmp requests against a D-Link device in your network.
ringStationControlTableSize
ring Station Control Table Size
1.3.6.1.2.1.16.10.1.1.2
The number of ringStationEntries in the ringStationTable associated with this ringStationControlEntry.
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IPHost Network monitor allows you to monitor ringStationControlTableSize on D-Link device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring D-Link switches right now.