to UNA,

Based on your suggestion, I modified tnsnames.ora

DBSP.WORLD =
(DESCRIPTION =
(FAILOVER=ON)
(ADDRESS = (PROTOCOL= TCP)(Host= purple)(Port= 1521))
(ADDRESS = (PROTOCOL= TCP)(Host= silver)(Port= 1521))
(CONNECT_DATA = (SID = DBU2))
)

then I issue

sqlplus system/manager@DBSP
> select * from v$instance;
select INSTANCE_NAME, HOST_NAME from v$instance;
INSTANCE_NAME HOST_NAME
---------------- ---------------------------------------------------------------
DBU2 silver


(still connect to the last one)

then I shutdown instance on silver, try to reconnect get
ORA-01034: ORACLE not available

What I did wrong?