Tips: Cluster communications

Consider these tips when you set up your communications paths.

• Be sure you have adequate bandwidth on your communication lines to handle the non cluster activity along with the clustering heartbeating function and continue to monitor for increased activity.
• For best reliability, do not configure a single communication path linking one or more nodes.
• Do not overburden the line that is responsible for ensuring that you are still communicating with a node.
• Eliminate as many single points of failure as possible, such as having two communication lines coming into a single adapter, same input-output processor (IOP), or same expansion unit.
• If you have an extremely high volume of data being passed over your communication lines, you may want to consider putting data replication and heartbeat monitoring on separate networks.
• User Datagram Protocol (UDP) multicast is the preferred protocol that the cluster communications infrastructure uses to send cluster management information between nodes in a cluster. When the physical media supports multicast capabilities, cluster communications uses the UDP multicast to send management messaging from a given node to all local cluster nodes that support the same subnet address. Messages that are sent to nodes on remote networks are always sent by using UDP point-to-point capabilities. Cluster communications does not rely on routing capability for multicast messages.
• The multicast traffic that supports cluster management messaging tends to fluctuate by nature. Depending on the number of nodes on a given LAN (that supports a common subnet address) and the complexity of the cluster management structure that is chosen by the cluster administrator, cluster-related multicast packets can easily exceed 40 packets per second. Fluctuations of this nature can have a negative effect on older networking equipment. One example is congestion problems on devices on the LAN that serve as Simple Network Management Protocol (SNMP) agents that need to evaluate every UDP multicast packet. Some of the earlier networking equipment does not have adequate bandwidth to keep up with this type of traffic. You need to ensure that you or the network administrator has reviewed the capacity of the networks to handle UDP multicast traffic to make certain that clustering does not have a negative effect on the performance of the networks.