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The following errors are generated in the events log showing interface and node with status unknown. When the interfaces and nodes are checked, the status changed to up.
Data Process Logs
Error: <2015-12-03 12:13:37,408 [STP ProcessingPipeLine Thread #914] DEBUG SolarWinds.Interfaces.Collector.DataProcessing.PollerResultPreProcessor -
PreProcessing data for entity I:514 poller I.Status.SNMP.IfTable (Index:1, State:Failed, PollTime:01/01/0001 00:00:00) jobResults: >
Error:<2015-11-09 13:56:40,799  ERROR Main - HandleRecv() unable to send request - Firing reply with error
Interface Collector Logs
2015-11-09 13:56:40,815 [STP SmartThreadPool Thread #78] ERROR Main - SNMPSocket::SendData() Socket Exception: HostUnreachable
2015-11-09 13:56:40,815 [STP SmartThreadPool Thread #78] ERROR Main - Unable to send packet!
2015-11-09 13:56:40,815 [STP SmartThreadPool Thread #78] ERROR Main - SNMPManager::InternalQuery() unable to send request
All NPM versions
This condition has been seen in environments that have deployed stateful firewalls. Stateful firewalls are usually configured to automatically drop packets with request-ids higher than a specified value. As two of its intended functions, Orion NPM makes ICMP and SNMP requests of monitored devices to acquire status and performance data, respectively. If these requests are made through a stateful firewall that is configured to drop packets with request-ids above a specified value, Orion NPM ICMP and SNMP requests with request-ids exceeding this configured value will be dropped.
In this event, Orion NPM will never receive a response from the monitored device, and it will consider the device status as "unknown". This condition is temporarily resolved by rebooting the Orion NPM server, as request-ids after a reboot are renumbered.
To correct this condition, modify the configuration of any firewall along the monitoring path between your Orion server and the devices reporting as "unknown" to allow higher request-ids.