What does Windows error code 5066 (ERROR_CLUSTER_LAST_INTERNAL_NETWORK) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The operation would remove the only internal communication network between active nodes.

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ERROR_CLUSTER_NODE_ALREADY_MEMBER	ERROR_CLUSTER_NETWORK_HAS_DEPENDENTS

ERROR_CLUSTER_LAST_INTERNAL_NETWORK

Failover Clustering blocks this change to protect membership communication. Removing the last internal path can partition the cluster and risk loss of quorum or split-brain protection behavior.

What to check

Identify all networks that allow node-to-node communication and confirm that at least one resilient path remains.
Add or repair a valid internal network before changing the role of the existing one.
Run validation after any planned network redesign, especially in a two-node cluster.

Get-ClusterNetwork; Get-ClusterNetworkInterface; Test-Cluster

Microsoft: Get-ClusterNetwork

Microsoft: Test-Cluster

Microsoft: Quorum in a failover cluster

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