The term "Cluster" from an IronPort perspective can be a bit confusing sometimes. From an IronPort perspective we use the term clustering to refer to centralized management. This is a feature that allows you to share a configuration across multiple appliances. If you are referring to load balancing using multiple appliances the IronPort Email Security Appliance does not have anything built in to configure for this. This is achieved through the use of DNS round robin.
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