12c: ORA-28040 After Upgrade: No Matching Authentication Protocol

12c: ORA-28040 After Upgrade: No Matching Authentication Protocol

After upgrade to the version 12c database, the following error 'ORA-28040: No matching authentication protocol exception' are thrown when attempting to connect from remote 

clients:] 

Version 12.1:

The default setting for the new parameters is 11.  Any client that attempts to connect must 

be at version 11 or higher unless these parameters are explicitly set in the server side sqlnet.ora file.

Version 12.2 note:

The default for the SQLNET.ALLOWED_LOGON_VERSION_SERVER setting has changed in 12.2 from 11 to 12.

See:  https://docs.oracle.com/database/122/DBSEG/configuring-authentication.htm#DBSEG33223

Important note for 12.2:  If your client is not at least 11.2.0.3 or includes the CPUOCT2012 patch you will not be able 

to use the 12 setting.

Typically, the sqlnet.ora file that would be referenced by the database is located in RDBMS_HOME/network/admin.

Soultion :

Set these parameters at the lowest version level that is required in your environment.  

For example:  All clients at version 10 or higher would require this setting:

 SQLNET.ALLOWED_LOGON_VERSION_SERVER=10

 SQLNET.ALLOWED_LOGON_VERSION_CLIENT=10

Note : SQLNET.ALLOWED_LOGON_VERSION_CLIENT would be necessary on the server when the database is 'acting' as a client.  Such as the case of a database link.

There is  no need to restart either the listener or the database after this change.  See additional notes below.