code: def add_intf_router(self, rout_id, tenant_id, subnet_lst):
          
          try:
              for subnet_id in subnet_lst:
                  body = {'subnet_id': subnet_id}
                  intf = self.neutronclient.add_interface_router(rout_id,
                                                                 body=body)
                  intf.get('port_id')
          except Exception as exc:
              LOG.error("Failed to create router intf ID %(id)s,"
                        " Exc %(exc)s", {'id': rout_id, 'exc': str(exc)})
              return False
          return True
docs: Add the interfaces to a router.