Oracle Data Guard Error DGMGRL for Linux: Version 12.1.0.2.0 - 64bit Production

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Connected as SYSDBA.
DGMGRL&gt; show configuration

Configuration - oradb

  Protection Mode: MaxPerformance
  Members:
  oradb    - Primary database
    Error: ORA-16778: redo transport error for one or more databases

    oradb_s2 - Physical standby database
      Warning: ORA-16857: standby disconnected from redo source for longer than specified threshold

Fast-Start Failover: DISABLED

Configuration Status:
ERROR   (status updated 1 second ago)

DGMGRL&gt;


is someone has any idea about the following error and warning?
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.