Configuring the Cluster’s Multicast Address

The installation process now sets this application’s multicast address automatically. Multicast must handle the communication between elements of a cluster--whether application or mediation servers. You can disable multicast discovery of servers (mediation server to application server) or clients (client to application server[s]) as described in Disabling Multicast.

The range for multicast as defined by the Host Extensions for IP Multicasting [RFC1112] is from 224.0.0.0 to 239.255.255.255. The following is an excerpt from the article Internet Multicast Addresses that should be considered when assigning addresses:

“The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive, is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols, such as gateway discovery and group membership reporting. Multicast routers should not forward any multicast datagram with destination addresses in this range.”

Use an address above 225.0.0.0, like the one cited in the example above.

Application servers in a cluster use multicast to determine when a peer has come up or when one goes down. This communication usually has to be done in a matter of seconds or even milliseconds and should probably be exclusive on the multicast channel chosen. In other words, best practice is to use a multicast address exclusively for a cluster. Do not use the same multicast address for two clusters and do not use the same multicast for other multicast communication.

If a cluster member asks a running peer to respond but the traffic on the channel prevents a response within the designated timeout, then the requesting member concludes the running peer is gone and takes appropriate fail-over action, even if the peer is running without problem. This can be the source of undesirable behavior from your cluster. For example, it could initiate the rule resynchronization process on the cluster.