Peer down handler.

        Cleans up the paths in global tables that was received from this peer.

def on_peer_down(self, peer):
        """Peer down handler.

        Cleans up the paths in global tables that was received from this peer.
        """
        LOG.debug('Cleaning obsolete paths whose source/version: %s/%s',
                  peer.ip_address, peer.version_num)
        # Launch clean-up for each global tables.
        self._table_manager.clean_stale_routes(peer)