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PRTG Manual: Install a PRTG Cluster
PRTG offers single failover clustering in all licenses—even using the freeware edition. A single failover cluster consists of two servers ("Current Master" Node and "Failover Node"), each of them running one installation of PRTG. They are connected to each other and exchange configuration and monitoring data. You can run a cluster with up to 5 nodes.
Click here to enlarge: http://media-s3.paessler.com.s3.amazonaws.com/prtg-screenshots/clustering-1_en.png
For detailed information, please see Failover Cluster Configuration .
Knowledge Base: What's the Clustering Feature in PRTG?