Q: What is the "relaxed" cluster heartbeat setting in Windows Server 2012 R2 clusters?
A: Cluster communications are very sensitive to latency. By default, every second (SameSubnetDelay) cluster nodes send heartbeats to each other, and if five heartbeats (SameSubnetThreshold) in a row aren't returned from a node, then that node is considered unavailable and removed from the cluster.
In many environments, it has been found that for Hyper-V clusters the network might not be as well-designed as would typically be the case for a cluster, and that five seconds might be to strict. Therefore as soon as a virtual machine (VM) role is created on a cluster in Windows Server 2012 R2, the cluster goes into a relaxed threshold mode (instead of the normal Fast Failover), which considers a node unavailable after 10 missed heartbeats instead of five (the maximum possible that can be configured is 120). The value can be viewed using Windows PowerShell:
(Get-Cluster).SameSubnetThreshold 10