RAC Password

Hi, 

how to change sys password in RAC ,does alter user must be issued in each instances?

For a password change activity i have change password in one instance using alter user and able to connect to all three instances from  client. 

but we  have a seperate password file in all the instances and  parameter remote_login_passwordfile  is set  as exclusive,i read that this  enables to change the SYS password with the ALTER USER command for only one instance

in my case it worked in  all three instances,please clarify why it happens also let me know if we have to change password in all the instances in RAC for password changes