备库不能用catalog注册。正确的方式如下：
链接主库注册，然后连到备库备份。

oracle@db03[/home/oracle]: rman target / catalog rman/rman@laa

Recovery Manager: Release 11.2.0.4.0 - Production on Mon May 15 09:36:52 2017
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
connected to target database: TESTDB (DBID=2725042839)
connected to recovery catalog database
RMAN> register database;
database registered in recovery catalog
starting full resync of recovery catalog
full resync complete
RMAN>CONFIGURE DB_UNIQUE_NAME 'TESTDB_STBY' CONNECT IDENTIFIER 'TESTDB_STBY';
RMAN> list db_unique_name of database;
List of Databases DB Key DB Name DB ID Database Role Db_unique_name
------- ------- ----------------- --------------- ------------------
1849 TESTDB 2725042839 PRIMARY TESTDB
1849 TESTDB 2725042839 STANDBY TESTDB_STBY
RMAN> resync catalog from db_unique_name all;
starting full resync of recovery catalog
full resync complete resyncing from database with DB_UNIQUE_NAME TESTDB_STBY

在主库连接到备库（adg）：
rman catalog rman/rman@laa
RMAN>create catalog; 成功
exit

主库上：
rman target / catalog rman/rman@laa
RMAN>register database; 成功
RMAN>resync catalog; 成功
RMAN> list db_unique_name of database;
List of Databases
DB Key DB Name DB ID Database Role Db_unique_name

1 test 885568389 PRIMARY test
1 test 885568389 STANDBY test_stby

这样也是可以的吧，所有的操作都在主库上。