TNS

--Parameters for multiple listener protocol addresses for service in tnsnames.ora
FAILOVER (Default - on) - if on - fail over to next listener if the first listener fails
LOAD_BALANCE (Default -off) - if on - random sequence


--client tracing
cat sqlnet.ora
TRACE_LEVEL_CLIENT=admin
--example
oracle@rac1:/tmp> mkdir tns
oracle@rac1:/tmp> cd tns
oracle@rac1:/tmp/tns> sqlplus -L t/t@rac1

SQL*Plus: Release 10.2.0.1.0 - Production on Thu Feb 23 16:30:51 2012

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

ERROR:
ORA-01017: invalid username/password; logon denied


SP2-0751: Unable to connect to Oracle.  Exiting SQL*Plus
oracle@rac1:/tmp/tns> ls
cli_31630.trc
 

--Startup via tns (ora-12514)

Solution use static database registration instead of dynamic in listener.ora,
restart listener
and
Connect as :
sqlplus sys@rac1 as sysdba