A feature called automatic Oracle instance registration is the ability of an instance to register itself automatically with the listener at startup. Before Oracle 8i, an instance had to be configured via the listener.ora file. Now Oracle provides automated instance registration with the associated listener. Database instance registration enables load balancing and failover features as well.
When started, the database will register itself with the default listener without any additional configuration requirements. If you use multiple listener configurations, then you will need to add some additional parameters to the parameter file or spfile file for that instance. Configuration for listeners other than the default listener will require the inclusion of the SERVICE_NAME and LOCAL_LISTENER optional parameters in the init.ora file of the instance you want registered. Note that the LOCAL_LISTENER parameter also overrides other parameters of Shared server configuration. This is most important thing and as Oracle DBA you should need to aware of this.
If you use multiple listeners, you will need to modify your parameter file or you will find your database registering itself with both the listener you have defined for it and the default listener.
