vSAN Datastore Inaccessible and vCenter Server Offline Due to MTU Mismatch.

Products

• VMware vSAN

Issue/Introduction

When experiencing network infrastructure or MTU configuration issues within a vSAN cluster, may observe the following symptoms:

• vCenter Server is inaccessible, and production virtual machines (VMs) are offline.

• The vSAN cluster is partitioned. All virtual machines residing on the vSAN datastore display an Inaccessible status in the inventory.

• esxcli vsan cluster get shows a "Sub-Cluster Member Count" of 1.
  
  
  • Cluster Information
   Enabled: true
   Local Node UUID: ######-####-####-####-############
   Local Node Type: NORMAL
   Local Node State: MASTER
   Local Node Health State: HEALTHY
   Sub-Cluster Master UUID: ######-####-####-####-############
   Sub-Cluster Backup UUID:
   Sub-Cluster UUID: ######-####-####-####-############
   Sub-Cluster Membership Entry Revision: 1
   Sub-Cluster Member Count: 1
   Sub-Cluster Member UUIDs: ######-####-####-####-############
   Sub-Cluster Member HostNames: ESX####
   Sub-Cluster Membership UUID: ######-####-####-####-############
   Unicast Mode Enabled: true
   Maintenance Mode State: OFF
   Config Generation: ######-####-####-####-############ 19 2026-06-10T21:53:53.251
   Mode: REGULAR
   vSAN ESA Enabled: false


• esxcli vsan debug object health summary get shows "Inaccessible" objects.
  
  Health Status                                              Number Of Objects
---------------------------------------------------------  -----------------
remoteAccessible                                                           0
inaccessible                                                             533
reduced-availability-with-no-rebuild                                       0
reduced-availability-with-no-rebuild-delay-timer                           0
reducedavailabilitywithpolicypending                                       0
reducedavailabilitywithpolicypendingfailed                                 0
reduced-availability-with-active-rebuild                                   0
reducedavailabilitywithpausedrebuild                                       0
data-move                                                                  0
nonavailability-related-reconfig                                           0
nonavailabilityrelatedincompliancewithpolicypending                        0
nonavailabilityrelatedincompliancewithpolicypendingfailed                  0
nonavailability-related-incompliance                                       0
nonavailabilityrelatedincompliancewithpausedrebuild                        0
healthy                                                                    0
  
  
• Standard pings (MTU 1500) between hosts succeed, but pings with jumbo frames (MTU 9000) fails
  
  vmkping -I vmkX ##.##.##.## -d -s 8972
PING ##.##.##.## (##.##.##.##): 8972 data bytes

--- ##.##.##.## ping statistics ---
3 packets transmitted, 0 packets received, 100% packet loss

Environment

VMware vSAN 8.x
VMware vCenter Server 8.x
vSphere Distributed Switch (vDS)

Cause

The issue occurs due to an MTU size mismatch between the vSphere Distributed Switch (vDS) and the underlying vSAN VMkernel adapters (vmk).

When the vDS MTU is reduced to 1500 while the vSAN VMkernel interfaces (vmk3) remain configured at 9000, the VMkernel interfaces continue to transmit jumbo frames. The vDS drops or improperly fragments these large frames, causing severe packet loss and a total breakdown of vSAN cluster communication. Because vSAN relies on consistent, low-latency heartbeat exchanges and metadata synchronization, this mismatch completely isolates the ESXi hosts from one another, partitioning the cluster and dropping the vSAN datastore offline.

Resolution

Restore connectivity by aligning the VMkernel MTU to the current switch setting via the CLI, then revert the configuration through the vCenter UI.

1. Update VMkernel MTU via ESX CLI:
   
      Log in to each ESXi host in the cluster via SSH.
      Identify the VMkernel adapter used for vSAN traffic: esxcli vsan network list
      Set the VMkernel MTU to 1500 to match the vDS setting (replace vmkx with your specific adapter, e.g., vmk3): esxcli network ip interface set -i vmkX -m MTU(1500/9000)
   
   
2. Check that the vSAN cluster is no longer partitioned and objects are healthy: esxcli vsan debug object health summary get
   
   
3. Power on the vCenter Server VM once the datastore is accessible.
   
   
4. Revert vDS MTU via UI:
   
      Log in to the vSphere Client.
      Navigate to Networking > Distributed Switch > Configure > Settings > Properties.
      Click Edit and change the MTU back to 9000.
   
   
5. Revert VMkernel MTU:
   
      Schedule a maintenance window to revert the VMkernel adapters (vmk) back to 9000 to match the vDS for optimal performance.

Additional Information

Configuring Standard vSwitch (vSS) or virtual Distributed Switch (vDS) from the command line in ESXi