This is more of a point to remember for myself.

In 11g, you don't perform the sys operations on the ASM instance with the sysdba privileges.

You have to use the sysasm privileges.

You can either assign a user sysasm privileges through the sqlplus or include a user at OS  level in OSASM group.

To start an ASM instance:

export ORACLE_SID+ASM
sqlplus '/ as sysasm'