This object specifies the loopback interface to be used
by GGSN as the charging source interface. When this is
set to any value other than -1, the GTP path to the
charging gateways will be re-created with the new address
obtained from the loopback interface and this address will
be used for all the charging messages in GGSN. When this
is set to -1, the global GTP virtual template will be used
as source interface for all charging messages.