code: def mgmt_nw_id(cls):
          
          if cls._mgmt_nw_uuid is None:
              tenant_id = cls.l3_tenant_id()
              if not tenant_id:
                  return
              net = bc.get_plugin().get_networks(
                  bc.context.get_admin_context(),
                  {'tenant_id': [tenant_id],
                   'name': [cfg.CONF.general.management_network]},
                  ['id', 'subnets'])
              if len(net) == 1:
                  num_subnets = len(net[0]['subnets'])
                  if num_subnets == 0:
                      LOG.error('The management network has no subnet. '
                                'Please assign one.')
                      return
                  elif num_subnets > 1:
                      LOG.info('The management network has %d subnets. The '
                               'first one will be used.', num_subnets)
                  cls._mgmt_nw_uuid = net[0].get('id')
                  cls._mgmt_subnet_uuid = net[0]['subnets'][0]
              elif len(net) > 1:
                  # Management network must have a unique name.
                  LOG.error('The management network for does not have '
                            'unique name. Please ensure that it is.')
              else:
                  # Management network has not been created.
                  LOG.error('There is no virtual management network. Please '
                            'create one.')
          return cls._mgmt_nw_uuid
docs: Returns id of the management network.