The value of iesIfFwdSubIf specifies the forwarding
subscriber interface for a subscriber interface in a
retailer context. This value can only be set at row
creation time along with iesIfFwdServId, and it is only
valid when iesIfType is 'subscriber'. The interface
pointed to by iesIfFwdSubIf MUST be of type 'subscriber'
in the the service context defined by iesIfFwdServId.