The number of the loopback interface, associated with the specified
virtual router, whose IP address is used as the source address when
transmitting IP packets on unnumbered remote access user links.
For example, if the associated loopback interface for this domain/router
was configured via the console as 'loopback 2', this object would
contain the integer value 2.
A value of -1 indicates the loopback interface is unspecified.