Enqueues Enhanced RR EOR if for given SOR a EOR is not already
        sent.

def _enqueue_eor_msg(self, sor):
        """Enqueues Enhanced RR EOR if for given SOR a EOR is not already
        sent.
        """
        if self._protocol.is_enhanced_rr_cap_valid() and not sor.eor_sent:
            afi = sor.afi
            safi = sor.safi
            eor = BGPRouteRefresh(afi, safi, demarcation=2)
            self.enque_outgoing_msg(eor)
            sor.eor_sent = True