ORA-16857: member disconnected from redo source for longer than specified threshold

-bash-4.2$ dgmgrl
DGMGRL> connect sys/a@db1
Connected to "db1"
Connected as SYSDBA.
DGMGRL> show configuration;

Configuration - dg_db1_config

  Protection Mode: MaxPerformance
  Members:
  db1    - Primary database
    Error: ORA-16778: redo transport error for one or more members

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

Fast-Start Failover: DISABLED

Configuration Status:
ERROR   (status updated 2 seconds ago)

DGMGRL> show database db1

Database - db1

  Role:               PRIMARY
  Intended State:     TRANSPORT-ON
  Instance(s):
    db1_1
    db1_2
      Error: ORA-16737: the redo transport service for member "stadby_db" has an error

Database Status:
ERROR

DGMGRL> show database stadby_db

Database - stadby_db

  Role:               PHYSICAL STANDBY
  Intended State:     APPLY-ON
  Transport Lag:      0 seconds (computed 175714 seconds ago)
  Apply Lag:          0 seconds (computed 175714 seconds ago)
  Average Apply Rate: 188.00 KByte/s
  Real Time Query:    OFF
  Instance(s):
    stadby_db_1 (apply instance)
    stadby_db_2

  Database Warning(s):
    ORA-16857: member disconnected from redo source for longer than specified threshold

Database Status:
WARNING

DGMGRL> disable database stadby_db;
Disabled.
DGMGRL> enable database stadby_db;
Enabled.
DGMGRL> show database stadby_db;

Database - stadby_db

  Role:               PHYSICAL STANDBY
  Intended State:     APPLY-ON
  Transport Lag:      1 day(s) 14 hours 47 minutes 10 seconds (computed 0 seconds ago)
  Apply Lag:          1 day(s) 18 hours 16 minutes 17 seconds (computed 0 seconds ago)
  Average Apply Rate: 189.00 KByte/s
  Real Time Query:    OFF
  Instance(s):
    stadby_db_1 (apply instance)
    stadby_db_2

  Database Warning(s):
    ORA-16853: apply lag has exceeded specified threshold
    ORA-16855: transport lag has exceeded specified threshold

Database Status:
WARNING


DGMGRL> show database stadby_db;

Database - stadby_db

  Role:               PHYSICAL STANDBY
  Intended State:     APPLY-ON
  Transport Lag:      0 seconds (computed 0 seconds ago)
  Apply Lag:          0 seconds (computed 0 seconds ago)
  Average Apply Rate: 254.00 KByte/s
  Real Time Query:    OFF
  Instance(s):
    stadby_db_1 (apply instance)
    stadby_db_2

Database Status:
SUCCESS

DGMGRL> show configuration;

Configuration - dg_db1_config

  Protection Mode: MaxPerformance
  Members:
  db1    - Primary database
    stadby_db - Physical standby database

Fast-Start Failover: DISABLED

Configuration Status:
SUCCESS   (status updated 52 seconds ago)

DGMGRL>