Unable to enable Auto Negotiation on vmnic.

Products

• VMware vSphere ESXi

Issue/Introduction

• NIC is 25 Gbps.
• Unable to select 25 Gbps as speed / duplex on vSphere Client or Host Client.
• Running esxcfg-nics vmnicX -a on ESXi CLI to enable auto negotiation completes successfully without Errors.
  esxcfg-nics vmnicX -a

Environment

VMware vSphere ESXi 8.0

Cause

The physical switch port connected to the ESXi host is configured with a fixed speed or has Auto Negotiation disabled at the physical layer.
For Auto Negotiation to succeed and for certain high-speed profiles (like 25G/100G) to be advertised correctly, both ends of the physical link must be set to Auto Negotiation.

Resolution

Check physical switch configuration to see if the interface settings is set to Auto Negotiation.