iSCSI VMK Retains MTU 1500 After Migration to a Distributed vSwitch (DVS) Configured for MTU 9000

Products

• VMware vSphere ESXi

Issue/Introduction

After migrating an iSCSI VMkernel adapter (VMK) to a vSphere Distributed Switch (DVS) configured with MTU 9000, the adapter continues to use an MTU of 1500 instead of inheriting the DVS configuration. This can result in MTU mismatches and potential performance impact on the storage network.

Environment

VMware vSphere ESXi 7.x / 8.x

Cause

When a VMkernel adapter is initially created with an MTU of 1500 on a standard switch (vSS) or DVS and is later moved to another DVS configured with a higher MTU (e.g., 9000), the adapter does not automatically inherit the new DVS MTU configuration. The VMK retains the previously configured MTU value of 1500.

Resolution

Note : Ensure DVS MTU is set to 9000 before following the below steps.

To ensure that the VMkernel adapter inherits the correct MTU from the DVS set with MTU 9000:

1. Remove the existing iSCSI VMkernel adapter (configured with MTU 1500).
   • In the vSphere Client, navigate to the host.
   • On the Configure tab, expand Networking and select VMkernel adapters.
   • Click the 3 dots next to the iSCSI VMKernel and Remove.
2. Recreate the VMkernel adapter on the DVS.
   • In the vSphere Client, navigate to the host.
   • On the Configure tab, expand Networking and select VMkernel adapters.
   • Click on Add Networking, Select VMKernel Network Adapter, Next.
   • Under Select an existing network, choose the iSCSI dv portgroup from the list, Next.
   • Under Port properties, Select Get MTU from switch >> This allows the VMkernel adapter to inherit the MTU value configured on the DVS (i.e 9000).
   • Configure IPv4 settings and Finish.
3. The newly created VMkernel adapter will now correctly reflect an MTU of 9000.

Additional Information

Note :

Instead of re-creating the VMkernel adapter, to change the MTU of the iSCSI VMK, the network port binding on the iSCSI storage adapter must first be removed (If used). After removing the binding, the MTU value can be modified.

Detailed instructions are provided here: Unable to change or set MTU value of vmkernel (vmk)bound to iSCSI adapter from vCenter GUI