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  @param cfg: cluster configuration

  @param netmask: netmask to be verified

    @type cfg: L{config.ConfigWriter}

    @param cfg: cluster configuration

    @type cfg: L{config.ConfigWriter}

    @param cfg: cluster configuration

  @staticmethod

  def _InstanceCommunicationDDM(cfg, instance_communication, instance):

    """Create a NIC mod that adds or removes the instance

    communication NIC to a running instance.

    The NICS are dynamically created using the Dynamic Device

    Modification (DDM).  This function produces a NIC modification

    (mod) that inserts an additional NIC meant for instance

    communication in or removes an existing instance communication NIC

    from a running instance, using DDM.

    @type cfg: L{config.ConfigWriter}

    @param cfg: cluster configuration

    @type instance_communication: boolean

    @param instance_communication: whether instance communication is

                                   enabled or disabled

    @type instance: L{objects.Instance}

    @param instance: instance to which the NIC mod will be applied to

    @rtype: (L{constants.DDM_ADD}, -1, parameters) or

            (L{constants.DDM_REMOVE}, -1, parameters) or

            L{None}

    @return: DDM mod containing an action to add or remove the NIC, or

             None if nothing needs to be done

    """

    nic_name = "%s%s" % (constants.INSTANCE_COMMUNICATION_NIC_PREFIX,

                         instance.name)

    instance_communication_nic = None

    for nic in instance.nics:

      if nic.name == nic_name:

        instance_communication_nic = nic

        break

    if instance_communication and not instance_communication_nic:

      action = constants.DDM_ADD

      params = {constants.INIC_NAME: nic_name,

                constants.INIC_MAC: constants.VALUE_GENERATE,

                constants.INIC_IP: constants.NIC_IP_POOL,

                constants.INIC_NETWORK:

                  cfg.GetInstanceCommunicationNetwork()}

    elif not instance_communication and instance_communication_nic:

      action = constants.DDM_REMOVE

      params = None

    else:

      action = None

      params = None

    if action is not None:

      return (action, -1, params)

    else:

      return None

    # add or remove NIC for instance communication

    if self.op.instance_communication is not None:

      mod = self._InstanceCommunicationDDM(self.cfg,

                                           self.op.instance_communication,

                                           self.instance)

      if mod is not None:

        self.op.nics.append(mod)