ORA-28575: unable to open RPC connection to external procedure agent

When you are getting "ORA-28575: unable to open RPC connection to externa
procedure agent" error when using external procedure call in your PL/SQL code,
please check that you have following entries (in bold & italics) in your listener.ora
file and tnsnames.ora file:

Listener.ora

LISTENER_L1 =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROCL1))
)
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(Host = 172.10.1.1)(PORT = 1526))
)
)
)
SID_LIST_LISTENER_L1 =
(SID_DESC =
(SID_NAME = PLSExtProc)
(ORACLE_HOME = /software/ora9)
(PROGRAM = extproc)
)
(SID_DESC =
(ORACLE_HOME = /software/ora9/)
(SID_NAME = testdb)
)
)

Tnsnames.ora

EXTPROC_CONNECTION_DATA =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROCL1))
)
(CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)
)
)