AFF-A300 node enters unknown state during ONTAP upgrade

• During a planned ONTAP cluster upgrade on the AFF-A300 system, node 2 entered an 'Unknown' state. 
  Cluster::> storage failover show             
               Takeover          
Node           Partner        Possible State Description  
-------------- -------------- -------- -------------------------------------
Clusterc1a
               Clusterc1b false    In takeover
Clusterc1b
               Clusterc1a -        Unknown
              
2 entries were displayed.
• The node became unresponsive and its Service Processor (SP)/Baseboard Management Controller (BMC) was inaccessible remotely.
• The node had an uptime of 699 days prior to the event.
• SP or BMC reports heartbeat stop and raise "Power Reset" event:

[IPMI.notice]: 9200 | c0 | OEM: ffff7000ff00 | ManufId: 150300 | SP Power Reset
[IPMI.notice]: 9300 | c0 | OEM: fcff70560000 | ManufId: 150300 | POS Register:  Power on Reset(Normal Power Cycle)

or

[IPMI.notice]: 00d4 | c0 | OEM: ffff7000ff00 | ManufId: 150300 | BMC Power Reset
[IPMI.notice]: 00d5 | c0 | OEM: fcff70560000 | ManufId: 150300 | POS Register: Power on Reset(Normal Power Cycle)

• Partner takeover due to missed heartbeat or lost communication

[node_name: cf_main: cf.fsm.takeover.noHeartbeat:alert]: Failover monitor: Takeover initiated after no heartbeat was detected from the partner node.

or

cf_fastTimeout: cf.ic.heartBeatFailed:error]: HA interconnect: Heartbeat failed

or

cf_takeover: callhome.sfo.takeover:alert]: Call home for CONTROLLER TAKEOVER COMPLETE AUTOMATIC - Communication Error

• No other suspicious error messages.