Have you verified that the TNSNAMES.ORA file is being used? To do this, issue "tnsping service_name" from the command line, where "service_name" is the service name you added to the config file. If that works, then your problem is most likely that the TNSNAMES.ORA file that you are using is not the same as the database is using. Database links require that the TNSNAMES.ORA file be in the ORACLE_HOME/network/admin directory. On my servers, I place this file in a different directory. To get database links to use this file, I have to create a shortcut (Windows) or a symbolic link (Unix) in the ORACLE_HOME/network/admin directory and point it to the other TNSNAMES.ORA file. This shortcut or link should have the name "tnsnames.ora."
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