code: def program_rtr_default_gw(self, tenant_id, rout_id, gw):
          
          args = ['route', 'add', 'default', 'gw', gw]
          ret = self.program_rtr(args, rout_id)
          if not ret:
              LOG.error("Program router returned error for %s", rout_id)
              return False
          return True
docs: Program the default gateway of a router.