Explanation/Reference:
There are three types of disk groups based on the ASM redundancy level.
he redundancy levels are:
* External redundancy
ASM does not provide mirroring redundancy and relies on the storage system to provide RAID functionality. Any write error cause a forced dismount of the disk group. All disks must be located to successfully mount the disk group.
* Normal redundancy
ASM provides two-way mirroring. By default all files are mirrored so that there are two copies of every data extent. A loss of one ASM disk is tolerated.
* High redundancy
ASM provides triple mirroring by default. A loss of two ASM disks in different failure groups is tolerated.
Reference: Oracle Database Storage Administrator's Guide, ASM Mirroring and Disk Group Redundancy