6.14.2. Multicast Configuration

If you do not specify a multicast address in the cluster configuration file, the Red Hat High Availability Add-On software creates one based on the cluster ID. It generates the lower 16 bits of the address and appends them to the upper portion of the address according to whether the IP protocol is IPv4 or IPv6:
  • For IPv4 — The address formed is 239.192. plus the lower 16 bits generated by Red Hat High Availability Add-On software.
  • For IPv6 — The address formed is FF15:: plus the lower 16 bits generated by Red Hat High Availability Add-On software.

Note

The cluster ID is a unique identifier that cman generates for each cluster. To view the cluster ID, run the cman_tool status command on a cluster node.
You can manually specify a multicast address in the cluster configuration file with the following command:
ccs -h host --setmulticast multicastaddress
Note that this command resets all other properties that you can set with the --setmulticast option to their default values, as described in Section 6.1.5, “Commands that Overwrite Previous Settings”.
If you specify a multicast address, you should use the 239.192.x.x series (or FF15:: for IPv6) that cman uses. Otherwise, using a multicast address outside that range may cause unpredictable results. For example, using 224.0.0.x (which is "All hosts on the network") may not be routed correctly, or even routed at all by some hardware.
If you specify or modify a multicast address, you must restart the cluster for this to take effect. For information on starting and stopping a cluster with the ccs command, see Section 7.2, “Starting and Stopping a Cluster”.

Note

If you specify a multicast address, make sure that you check the configuration of routers that cluster packets pass through. Some routers may take a long time to learn addresses, seriously impacting cluster performance.
To remove a multicast address from a configuration file, use the --setmulticast option of the ccs but do not specify a multicast address:
ccs -h host --setmulticast