As a DBA, controlling the environment is key to preventing disruptive events during migrations. Using the database parameter, LOCAL_LISTENER for each ORACLE_SID on a separate port allows the DBA to turn off or on access to that database without affecting other ports and/or ORACLE_SID(s). If you use port 1521, dynamic registration will happen for each instance on that node, and hence that port is avoided to maintain control.
Oracle recommends having multiple listeners running in multiple Oracle Homes (see Note: 429074.1), but experience has taught me that the highest version listener executable will work for any single-version, down-level database installed on a single node while preventing conflicts. You can use any combination of multiple ports and multiple listeners, but the executable comes out of only a single ORACLE_HOME location, as shown in the following listing of Unix processes:
The key to using a higher level listener is the variable TNS_ADMIN, which overrides...
