iSCSI storage connectivity issues or slow boot due to MTU mismatch on ESXi

Products

• VMware vSphere ESXi

Issue/Introduction

• Datastores are not visible in the vSphere Client.
• ESXi host experiences slow boot or hangs at "Activating: system storage".
• LUNs are visible in storage devices but cannot be consumed as datastores.
• vmkping fails when using large packet sizes.

Environment

• VMware vSphere ESXi
• Software iSCSI Initiator
• Jumbo Frames (MTU 9000) configuration

Cause

An MTU mismatch exists in the data path. This typically occurs when the ESXi VMkernel port is configured for Jumbo Frames (MTU 9000) but the physical switch or storage array is configured for a standard MTU (1500), leading to dropped packets.

Resolution

1. Verify network connectivity using vmkping.
2. Test standard MTU size: vmkping -I vmkX <Target_IP>
3. Test Jumbo Frame connectivity (if applicable): vmkping -I vmkX <Target_IP> -d -s 8972
4. If the standard ping succeeds but the Jumbo Frame ping fails, an MTU mismatch exists.
5. Align the MTU settings across the entire path (NIC, vSwitch, Physical Switch, and Storage Array).
6. If the physical network does not support Jumbo Frames, revert the VMkernel and vSwitch MTU to 1500:
   • Navigate to Networking > VMkernel adapters.
   • Select the affected adapter and click Edit.
   • Change MTU to 1500.
7. Rescan storage adapters to mount the missing datastores.