How Clustering Works
Chapter 5 – Clustering Terms
A cluster node is a server within a cluster group. A cluster node can be Active or it can be Passive as per SQL Server Instance installation.
Heartbeat The heartbeat is a checkup mechanism arranged between two nodes using a private network set up to see whether a node is up and running. This occurs at regular intervals known as time slices. A failover is initiated, if heartbeat is not functioning, and another node in the cluster will take over the active resources.
The Private Network is available among cluster nodes only. Every node will have a Private Network IP address, which can be ping from one node to another. This is to check the heartbeat between two nodes.
The Public Network is available for external connections. Every node will have a Public Network IP address, which can be connected from any client within the network.