i got esxi host discovered by ome but it is not sending out a host down alert when the host is rebooted. any idea? I already configured the snmp according to this document
The server being restarted does not generate an error, so an alert is not created. If you want an alert to take place then you will want to set up a heartbeat. I'm not sure of all of the steps necessary, but I think this is something that is configured through ESXi. OME should be able to monitor the trap.
Here is a thread discussing the heartbeat traps. It should give you a good starting point of how to set it up:
restarting the ome server seems to have fix the issue