Oracle Data Guard Error
ORA-12700 to ORA-19400


  ORA-16857: standby disconnected from redo source for longer than specified threshold
  
  Cause: The amount of time the standby was disconnected from the redo source database exceeded the value specified by the
  'TransportDisconnectedThreshold' database property. It is caused by no
  network connectivity between the redo source and the standby
  databases.
  
  Action: Ensure that there is network connectivity between the redo source and standby databases, and the redo source is working properly.