" _DGB ":的做用是dgmrl直接通信用的。
"_DGMGRL" :静态监听的作用：

A static entry is needed for the Broker to be able to connect remotely to a database that has been shut down by the Broker during certain operations. They are required for the Broker to complete the following operations:

Switchover from the Primary database to a Physical Standby database
Reinstatement of failed Primary database after a manual or automatic failover has occurred.
Conversion of a Snapshot Standby database back to a Physical Standby database

To access a database that has been shutdown in one of the above situations, the Broker uses a default name for the static entry using the information from the LOCAL_LISTENER parameter of the instance and the keyword "_DGMGRL" and stores that information in the broker StaticConnectIdentifier property associated with each instance of the database. As a best practice, users should not change the default values assigned to the Broker StaticConnectIdentifier property or to the database LOCAL_LISTENER initialization parameter as the value of the StaticConnectIdentifier property will be automatically updated by the Broker whenever the instance specified by the SidName property is started on a different host.

Note: Static entries are only used by the Broker itself via the Broker defined StaticConnectIdentifier property. They are NEVER to be used for any other purpose by the user. This includes using it for the normal DGConnectIdentifier, RMAN connections or anything else. Also note that you must never define a static entry for the Broker 'DGB' service. That service is for the Broker exclusive use. Also note that using the "_DGMGRL" or "_DGB" in your TNSNAMES definition is expressly forbidden.

================

2 Oracle Data Guard Installation

To enable DGMGRL to restart instances during the course of broker operations, a static service must be registered with the local listener of each instance. A static service registration is also required to enable the observer to restart instances as part of automatic reinstatement of the old primary database after a fast-start failover has occurred. By default, the broker assumes a static service name of db_unique_name_DGMGRL.db_domain and expects the listener has been started with the following content in the listener.ora file:

LISTENER = (DESCRIPTION =
     (ADDRESS_LIST=(ADDRESS=(PROTOCOL=tcp)(HOST=host_name)
     (PORT=port_num))))
SID_LIST_LISTENER=(SID_LIST=(SID_DESC=(SID_NAME=sid_name)
     (GLOBAL_DBNAME=db_unique_name_DGMGRL.db_domain)
     (ORACLE_HOME=oracle_home)
     (ENVS="TNS_ADMIN=oracle_home/network/admin")))

_DGMGRL and _DGB

2 Oracle Data Guard Installation