[oracle@node1 ~]$ sqlplus hr/hr@standby
SQL*Plus: Release 10.2.0.1.0 - Production on Wed Aug 4 17:50:17 2010
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> create database link mylink using 'standby';
Database link created.

SQL> select count(*) from employees@mylink;
COUNT(*)
----------
107

SQL> conn / as sysdba
Connected.

SQL> select name from v$database;
NAME
---------
STANDBY

SQL> create database link mylink1 using 'primary';
Database link created.

SQL> select count(*) from employees@mylink1;

COUNT(*)
----------
107

[oracle@node1 ~]$ export ORACLE_SID=primary

[oracle@node1 ~]$ sqlplus '/ as sysdba'
SQL*Plus: Release 10.2.0.1.0 - Production on Wed Aug 4 18:24:33 2010
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> select name from v$database;
NAME
---------
PRIMARY

SQL> create table hr.sir(name varchar2(30));
Table created.

SQL> insert into hr.sir values('mysir');
1 row created.

SQL> commit;
Commit complete.

SQL> select * from hr.sir;
NAME
------------------------------
mysir

SQL> commit;
Commit complete.

SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
- Production
With the Partitioning, OLAP and Data Mining options

[oracle@node1 ~]$ export ORACLE_SID=standby

[oracle@node1 ~]$ sqlplus '/ as sysdba'
SQL*Plus: Release 10.2.0.1.0 - Production on Wed Aug 4 18:27:02 2010
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> select name from v$database;
NAME
---------
STANDBY

SQL> conn hr/hr@standby
Connected.

SQL> select count(*) from sir@mylink1;
COUNT(*)
----------
1

SQL> select * from hr.sir@mylink1;
NAME
------------------------------
mysir