How many listeners can a instance register with?

Question

when we create a second listener, we must register it using the following commands:
sql>alter system set local_listener='list2';
sql>alter system register;
But, after this, will the first listener still registered? and do we need to set the local_listener back by:
SQL> alter system set local_listener='';
SQL> alter system register;
if only one listener can be registered at a time, what's the point to have a second listener?
thanks!

Answer 1

An Oracle Instance can be registered to more than one listener. (http://download-east.oracle.com/docs/cd/B14117_01/server.101/b10755/initparams096.htm)
It is useful in case of failure, and load balancing.
It has to be defined with the local_listener initialization parameter:
LOCAL_LISTENER=local_listener_entry_at_tnsnames
At the tnsnames, declare an entry with all of the listener's address list.
LOCAL_LISTENER_ENTRY_AT_TNSNAMES=
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = alpha.us.oracle.com)(PORT = 1521))
(ADDRESS = (PROTOCOL = TCP)(HOST = alpha.us.oracle.com)(PORT = 1525))
At the SQL prompt, issue the command:
ALTER SYSTEM SET LOCAL_LISTENER=LOCAL_LISTENER_ENTRY_AT_TNSNAMES;
ALTER SYSTEM REGISTER;
At the OS prompt: Issue lsnrctl services <name of first listener> and lsnrctl services <name of second listener>
and you'll see your instance will be registered to both listeners.
So you don't have to change back