code: def _cmp_by_cluster_list(path1, path2):
      
      def _get_cluster_list_len(path):
          c_list = path.get_pattr(BGP_ATTR_TYPE_CLUSTER_LIST)
          if c_list is None:
              return 0
          else:
              return len(c_list.value)
  
      c_list_len1 = _get_cluster_list_len(path1)
      c_list_len2 = _get_cluster_list_len(path2)
      if c_list_len1 < c_list_len2:
          return path1
      elif c_list_len1 > c_list_len2:
          return path2
      else:
          return None
docs: Selects the route received from the peer with the shorter
      CLUSTER_LIST length. [RFC4456]
  
      The CLUSTER_LIST length is evaluated as zero if a route does not
      carry the CLUSTER_LIST attribute.