def source_interface
      return nil unless Feature.tacacs_enabled?
      i = config_get('tacacs_global', 'source_interface')
      i.nil? ? default_source_interface : i.downcase
    end
def source_interface
      return nil unless Feature.tacacs_enabled?
      i = config_get('tacacs_global', 'source_interface')
      i.nil? ? default_source_interface : i.downcase
    end
Get source interface