ORA-15235: Diskgroup Mount Mode Conflicts With Existing Mount

This is because, the diskgroup is dismounted in local node, but it is in mount stage is remote nodes.

Solution :

SQL> select inst_id,name,state from gv$asm_diskgroup where name='DATA';

INST_ID NAME STATE
---------- ------------------------------ -----------
1 DATA DISMOUNTED
2 DATA MOUNTED

So to mount an asm diskgroup in restrict mode, Make sure the diskgroup is dismounted on all remote nodes. NODE 2 :

alter diskgroup DATA dismount;

Now connect to NODE 1 and run the restrict command.

SQL> alter diskgroup DATA mount restricted;

Diskgroup altered.