code: def _add_default_tz_bindings(self, context, switch, network_id):
          
          default_tz = CONF.NVP.default_tz
  
          # If there is no default tz specified it's pointless to try
          # and add any additional default tz bindings.
          if not default_tz:
              LOG.warn("additional_default_tz_types specified, "
                       "but no default_tz. Skipping "
                       "_add_default_tz_bindings().")
              return
  
          # This should never be called without a neutron network uuid,
          # we require it to bind some segment allocations.
          if not network_id:
              LOG.warn("neutron network_id not specified, skipping "
                       "_add_default_tz_bindings()")
              return
  
          for net_type in CONF.NVP.additional_default_tz_types:
              if net_type in TZ_BINDINGS:
                  binding = TZ_BINDINGS[net_type]
                  binding.add(context, switch, default_tz, network_id)
              else:
                  LOG.warn("Unknown default tz type %s" % (net_type))
docs: Configure any additional default transport zone bindings.