Run the tnsping command from the source and destination servers.

[oracle @ ora12c-prm admin] $ tnsping standby-db-unique-name
[oracle @ ora12c-dup admin] $ tnsping primary-db-unique-name

Oracle recommends that you connect remotely as sysdba to ensure that the communication is established properly.

Start SQL*Plus using a command in the following format:

sqlplus {username | /} [as sysdba]

For example:

$ sqlplus / AS SYSDBA
Enter password: password

Alternatively, to start SQL*Plus connected to a database other than the default, enter the SQL*Plus command in the format:

$> sqlplus username/password@connect_identifier