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When a user encounters an error, it is usually indicated as a two-part error string such as "TNS-12550". The prefix to an Oracle error message, "TNS" in this example, specifies the source of the error message. Oracle calls this the facility. The suffix, "12550", is the error number. Together they allow you look up information on the error condition itself. Determining the source of the error is the first step in selecting the proper tool to diagnose the problem.
Note: Be aware, the source of the error message is not necessarily the source of the error. In fact, the facility merely specifies the Oracle software component that determined something was wrong and that was first able to notify you of the error. Often, you will receive a list of errors at one time. This is called the error stack. Proceeding down the error stack, you see the facilities that encountered errors in the order they were detected, first to last down the list.
Given a facility and error number, the oerr utility will display the complete error message text and, often, the source of the error and recommended steps for resolving the problem.
Once you have found out more about the error from OERR, Instant Messages, or the Oracle Error Messages documentation, you employ one of the tools available for diagnosing connectivity problems in an Oracle network. These utilities are:
Click on the links above for details on each tool as well as when it should be used and how to use it.