Why is Multicast Preferred over Broadcast or Unicast Mode in Red Hat Enterprise Linux High Availability Clusters?

Environment

• Red Hat Enterprise Linux 6 (High Availability)
• Red Hat Enterprise Linux 5 (Clustering)

Issue

• What are the benefits of using multicast for cluster communication?

Resolution

• There are several reasons that broadcast mode can be less preferable for clusters. One issue is performance:
  • Broadcast mode is not suitable for deployments that require high performance locking. Such as GFS, GFS2, cmirror, any use of openais or corosync outside of RHEL that would require high performance message transport, such as Apache Qpid messaging.
  • In broadcast mode the packets are sent to the network's broadcast address and received by all nodes on the network. The nodes must receive and ignore packets that aren't intended for them. This creates a great deal of unnecessary traffic on the network, and adds to the processing load of the nodes.
  • In unicast mode UDP packets are sent directly to targeted cluster nodes. The packets that would have been broadcast or multicasted to all cluster nodes now have to be sent N-1 times (where N is the number of cluster nodes in the cluster) so there is a traffic increase compared to the other systems, adding to the processing load of the node
  • Multicast allows us to offload all of this load to the switch. This means less processing for the cluster nodes in terms of sending and receiving packets and less load on the network as packets are only sent to the destinations that need them.
• GFS and GFS2 also puts additional load on the network in the form of DLM messages that must go between the cluster nodes to manage filesystem locks. This traffic, combined with the other traffic necessary for cluster communication can overload the network or the cluster nodes.
• For more information on cluster communication see What are the different methods used for communication with RHEL High Availability?

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