Can multicast address be changed in Red Hat High Availability Cluster?

Environment

• Red Hat Enterprise Linux Server 5 (with the High Availability and Resilient Storage Add Ons)
• Red Hat Enterprise Linux Server 6 (with the High Availability and Resilient Storage Add Ons)

Issue

• Can multicast address be changed in Red Hat High Availability Cluster?
• How to change the multicast IP address in Red Hat cluster?

Resolution

• Multicast address can be changed from the multicast tab from the conga interface or in system-config-cluster(RHEL5 only).
• Make sure it is not blocked in the network.
• After the change is made in cluster.conf and propagated to all nodes, then cluster should be stop then restarted. All nodes should stop cluster software and then after all nodes have stopped the cluster software then restart the cluster software. Changing the mutlicast address for the cluster requires a full restart of cluster.
• Multicast IP addresses have a specific range and valid IP range is: 224.0.0.0 to 239.255.255.255. Using 224.0.0.0 is usually not a good ideas since some applications default to that.

Comments
The multicast tag can also be changed manually be editing the file /etc/cluster/cluster.conf. If the file is manually edited to add in the <multicast> section to the <cman> tag then, also increase the config_version int by 1 and propagate the changes across all the nodes of the cluster.

• Here is some information from the man page for cman about multicast:

        Multicast network configuration
               cman uses multicast UDP packets to communicate with  other
               nodes  in the cluster.  By default it will generate a mul-
               ticast address using 239.192.x.x where x.x  is  the  16bit
               cluster  ID number split into bytes. This, in turn is gen-
               erated from a hash of the cluster name though  it  can  be
               specified explicitly. The purpose of this is to allow mul-
               tiple clusters to share the same subnet - they  will  each
               use  a different multicast address. You might also/instead
               want to isolate clusters using the port  number  as  shown
               above.

It is possible to override the multicast address by specifying it in cluster.conf as shown:

                 <cman>
                     <multicast addr="229.192.0.1"/>
                 </cman>

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