After upgrading to ESXi 8.0, the Mellanox vmnic link either flaps continuously or stays down for a long time.

Products

• VMware vSphere ESXi

Issue/Introduction

Post-upgrade from ESXi 7.0 to 8.0, physical Mellanox ConnectX-5 and ConnectX-6 adapters may experience link instability. Symptoms include persistent link down notifications and switch ports reporting a non-connected state despite successful host-side driver initialization.
This behavior occurs due to physical layer negotiation conflicts between the adapter and the network switch. This article outlines the identification and resolution steps for this condition, specifically addressing Forward Error Correction (FEC) negotiation mismatches.

Symptoms:

• Inconsistent vmnic link state following ESXi host reboot.
• vmkernel logs display repetitive link down notifications for the impacted vmnic.
• Switch port interface shows a non-connected state.
• esxcli network nic get output indicates Link Status: Down.
• Host-side nmlx5_core driver and PCIe initialization complete without error.

Environment

• VMware vSphere ESXi 8.0 and higher.
• Mellanox ConnectX-5 / ConnectX-6 adapters

Cause

 A Forward Error Correction (FEC) negotiation mismatch exists between the Mellanox adapter profile and the physical network switch port configuration.

Resolution

1. Review the physical switch configuration logs to confirm the FEC mode (e.g., Reed-Solomon FEC) currently assigned to the port connected to the impacted vmnic.
2. Consult HPE customer advisory a00148079en_us to verify the compatible FEC settings for the Mellanox adapter firmware version installed on host ####.
3. Modify the physical switch port FEC configuration to align with the adapter profile requirements.
4. Perform a cold reboot of the ESXi host.
5. Verify link stability using the following command: esxcli network nic get -n vmnic#
6. Confirm the Link Status reports Up and traffic flows as expected across the uplink.

Additional Information

• Reference HPE Customer Advisory: https://support.hpe.com/hpesc/public/docDisplay?docId=a00148079en_us