def as_tree(self, level=0):
        """
        Display the routing tree representation in string
        format
        
        :rtype: str
        """
        ret = '--' * level + repr(self) + '\n'
        for routing_node in self:
            ret += routing_node.as_tree(level+1)
        return ret
Display the routing tree representation in string
        format
        
        :rtype: str