How to upgrade 11g standby database

I upgrade our production database from 10g into 11g, the production database is successful in 11g.

On standby server, I installed oracle 11g there, edit init file with compatible='11.2.0.3' and copy to the new 11g oracle_home. Then I ran the following commands with no errors.

SQL> connect / as sysdba
Connected to an idle instance.
SQL> startup nomount
ORACLE instance started.

Total System Global Area 2037747712 bytes
Fixed Size                  2222920 bytes
Variable Size             738198712 bytes
Database Buffers         1291845632 bytes
Redo Buffers                5480448 bytes
SQL> alter database mount standby database;

Database altered.

But when I check new alert log, I found ORA in the alert log