code: def mgmt_sec_grp_id(cls):
          
          if not extensions.is_extension_supported(bc.get_plugin(),
                                                   "security-group"):
              return
          if cls._mgmt_sec_grp_id is None:
              # Get the id for the _mgmt_security_group_id
              tenant_id = cls.l3_tenant_id()
              res = bc.get_plugin().get_security_groups(
                  bc.context.get_admin_context(),
                  {'tenant_id': [tenant_id],
                   'name': [cfg.CONF.general.default_security_group]},
                  ['id'])
              if len(res) == 1:
                  sec_grp_id = res[0].get('id', None)
                  cls._mgmt_sec_grp_id = sec_grp_id
              elif len(res) > 1:
                  # the mgmt sec group must be unique.
                  LOG.error('The security group for the management network '
                            'does not have unique name. Please ensure that '
                            'it is.')
              else:
                  # Service VM Mgmt security group is not present.
                  LOG.error('There is no security group for the management '
                            'network. Please create one.')
          return cls._mgmt_sec_grp_id
docs: Returns id of security group used by the management network.