All Barracuda Spam Firewall models 400 and above, all firmware versions.
Clustering Barracuda Spam Firewalls is not the same as load balancing them. Clustering will sync configuration, consolidate the Message Log, and back up individual user quarantine information onto a second Barracuda Spam Firewall, but clustering does not provide load balancing.
Using MX records with DNS round-robin is not a reliable way to achieve load balancing either. While setting up multiple MX records with equal priority might seem like a way to direct roughly equal amounts of traffic to each Barracuda Spam Firewall, it won't reliably work because most sending servers cache DNS records and will continue to send to the same server.
For load balancing, Barracuda Networks recommends the Barracuda Load Balancer, where mail traffic would be sent to a virtual IP address and be distributed to multiple, clustered Barracuda Spam Firewalls behind it. This will achieve both load balancing and high availability, should one or more Barracuda Spam Firewalls become unavailable.
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