code: def add_interface(self, interface):
          
          self.make_request(
              EngineCommandFailed,
              method='create', 
              href=self.get_relation(interface.typeof),
              json=interface)
          self._del_cache()
docs: Add interface is a lower level option to adding interfaces directly
          to the engine. The interface is expected to be an instance of
          Layer3PhysicalInterface, Layer2PhysicalInterface, TunnelInterface,
          or ClusterInterface. The engines instance cache is flushed after
          this call is made to provide an updated cache after modification.
          
          .. seealso:: :class:`smc.core.engine.interface.update_or_create`
          
          :param PhysicalInterface,TunnelInterface interface: instance of
              pre-created interface
          :return: None