vSAN Skyline Health: Basic unicast connectivity check failure or cluster partition

Products

• VMware vSAN

Issue/Introduction

• The "vSAN: Basic (unicast) connectivity check" reports a Red/Failed status in Skyline Health, and the cluster may show as partitioned.
• Skyline Health reports "vSAN cluster partition".
• esxcli vsan cluster get shows a "Sub-Cluster Member Count" lower than the total number of hosts.
• vSAN objects report status: reduced-availability-with-no-rebuild.
• A host may appear stuck in "Entering Maintenance Mode" indefinitely.

Environment

• VMware vSAN 7.x
• VMware vSAN 8.x
• VMware Cloud Foundation (VCF)

Cause

Network connectivity disruption on the vSAN VMkernel interface (e.g., vmk#) prevents unicast communication between the isolated node and the cluster master. This can be caused by physical link failures or switch-side configuration changes.

[root@esxi01:~] esxcli vsan cluster get
Cluster Information
   Enabled: true
   Current Local Time: 2026-05-20T17:10:57Z
   Local Node UUID: ########-####-####-####-############
   Local Node Type: NORMAL
   Local Node State: BACKUP
   Local Node Health State: HEALTHY
   Sub-Cluster Master UUID: ########-####-####-####-############
   Sub-Cluster Backup UUID: ########-####-####-####-############
   Sub-Cluster UUID: ########-####-####-####-############
   Sub-Cluster Membership Entry Revision: 20
   Sub-Cluster Member Count: 3
   Sub-Cluster Member UUIDs: ########-####-####-####-############, ########-####-####-####-############, ########-####-####-####-############
   Sub-Cluster Member HostNames: esxi01, esxi02, esxi03
   Sub-Cluster Membership UUID: ########-####-####-####-############
   Unicast Mode Enabled: true
   Maintenance Mode State: ENTERING
   Config Generation:########-####-####-####-############ # ########-####-####
   Mode: REGULAR
   vSAN ESA Enabled: false

Resolution

1. Identify the isolated node by running esxcli vsan cluster get on all hosts.

2. Identify the specific vSAN VMkernel interface being used

 
esxcli vsan network list

3. Verify connectivity from a healthy node to the isolated node using the vSAN VMkernel interface

vmkping -I vmk# -d -s 1472 <Isolated_Host_IP>

4. If the physical uplink is unresponsive, toggle the administrative state:

esxcli network nic down -n <vmnic#>
esxcli network nic up -n <vmnic#>

5. If a host is stuck in maintenance mode due to the partition, cancel the task

esxcli vsan maintenancemode cancel

6. Run esxcli vsan cluster get again to verify the Sub-Cluster Member Count matches the total host count.

[root@esxi01:~] esxcli vsan cluster get
Cluster Information
   Enabled: true
   Current Local Time: 2026-05-20T17:10:57Z
   Local Node UUID: ########-####-####-####-############
   Local Node Type: NORMAL
   Local Node State: BACKUP
   Local Node Health State: HEALTHY
   Sub-Cluster Master UUID: ########-####-####-####-############
   Sub-Cluster Backup UUID: ########-####-####-####-############
   Sub-Cluster UUID: ########-####-####-####-############
   Sub-Cluster Membership Entry Revision: 20
   Sub-Cluster Member Count: 4
   Sub-Cluster Member UUIDs: ########-####-####-####-############, ########-####-####-####-############, ########-####-####-####-############
   Sub-Cluster Member HostNames: esxi01, esxi02, esxi03
   Sub-Cluster Membership UUID: ########-####-####-####-############
   Unicast Mode Enabled: true
   Maintenance Mode State: ENTERING
   Config Generation:########-####-####-####-############ # ########-####-####
   Mode: REGULAR
   vSAN ESA Enabled: false