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IPMI: "Node busy" - ipmitool - Zabbix - max sessions to iDRAC6


from time to time I got the following error message when querying sensor / sdr informations from the iDRAC6 on R610-11g and R710-11G.

Get Session Challenge command failed: Node busy 
Error: Unable to establish LAN session 
Get Device ID command failed 
Unable to open SDR for reading 

I updated iDRAC6 firmware from 1.71 to 1.80 and 1.85 but the error still occurs. 

I also use Zabbix Network Monitoring for monitoring several sensors of these servers. Zabbix log mentions "network errors" and tries to reconnect 15 sec later.

I tried to figure out where this problem comes from and also posted in Zabbix Support forum: https://support.zabbix.com/browse/ZBX-3243

In the iDRAC Webinterface I saw under "iDRAC settings -> Properties -> iDRAC-Informations -> Integrated Dell Remote Access Controller 6 - Enterprise"

the attribute "Number of possible active sessions" has a value of "5". 

Does this mean that ONLY five simultanious sessions via IPMI (Lan) to the BMC are possible ?

Network monitoring systems try to query many sensor data in parallel. This restriction would make it impossible to query  data in nearly-real-time.

On the other hand the NMS should only establish one session and query the sensor data in this single session.

Is there a workaround or is it possible to change the "max sessions" value ?