How to change cluster from broadcast mode to multicast in Red Hat Enterprise Linux?

Solution Unverified - Updated -

Environment

  • Red Hat Enterprise Linux Server 5 (with the High Availability Add on)
  • Red Hat Enterprise Linux Server 6 (with the High Availability Add on)

Issue

  • Currently the cluster is running in broadcast mode and it needs to switch to multicast mode. Below is the cluster.conf:
<cluster config_version="34" name="test-cluster">
        <clusternodes>
                <clusternode name="node1" nodeid="1">
                        <fence>
                                <method name="Method1">
                                        <device ipaddr="node1" name="fence_node1"/>
                                </method>
                        </fence>
                </clusternode>
                <clusternode name="node2" nodeid="2">
                        <fence>
                                <method name="Method1">
                                        <device ipaddr="node2" name="fence_node2"/>
                                </method>
                        </fence>
                </clusternode>
        </clusternodes>
        <cman expected_votes="1" two_node="1" broadcast="yes"/>
        <fencedevices>
. .
        </fencedevices>
        <rm>
                <failoverdomains>
                        <failoverdomain name="node1-prefer" nofailback="1" ordered="1">
. . 
                        </failoverdomain>
                </failoverdomains>
                <resources>
                        <ip address="10.10.254.1" sleeptime="5"/>
                </resources>
                <service domain="node1-prefer" name="floatingIP" recovery="relocate">
                        <ip ref="10.10.254.1"/>
                </service>
        </rm>
</cluster>
  • What needs to be changed to do so?

Resolution

The default mode is multicast. There are two ways to change the broadcast cluster communication mode to multicast. Through command line or through Luci GUI interface. The steps would be pretty simple.

  • Using command line mode:

1. Remove 'broadcast="yes"' tab from 'cman' stanza and increment the config_version by 1 number.
2. Propagate the cluster.conf on another cluster node. Refer How can I propagate changes I've made to /etc/cluster/cluster.conf to all the nodes in my cluster? for more details.
3. Reboot both cluster nodes or stop/start cluster services for changes to take effect. Refer Which configuration changes require a restart of the entire cluster? for more details.

  • Using luci GUI interface:

1. login to luci
2. Click on "cluster" tab at the top
3. Click on cluster name
4. Click on "Multicast" tab
5. Set to "Use Multicast" radio button.
6. Click apply
7. Perform a cold cluster restart for changes to take effect.

This should change the broadcast mode to multicast.

Root Cause

  • The default mode for communication is multicast. If the broadcast tab in cman stanza removed, the cluster would switch to multicast mode.
  • If you'd like to change back to multicast mode refer to this article.

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