Network adapter (vmnic) shows as down in vCenter and does not connect

Products

• VMware vSphere ESXi

Issue/Introduction

• In vCenter, when viewing the ESXi host, under Configure, Virtual Switches, under Physical adapters, the vmnics Link Status show as Down, but the Admin status show as Up.
• Cable is properly connected to the interfaces and the switchport is active on the physical switch.
• Issue is the same after physically removing and reseating the network interface.

Environment

VMware vSphere ESXi 8.0

Cause

• Driver and Firmware Mismatches: ESXi 8.0 relies exclusively on native component drivers. Running an incompatible or outdated combination of native driver and device firmware can prevent the link from initializing correctly.
  
  
• • Refer to the articles for more information Supported drivers and firmware versions for I/O devices and Finding ESXi Drivers in the Broadcom Support Portal
    
    

• Unsupported Optics or SFP/QSFP Modules: If utilizing fiber optics or Direct Attach Copper (DAC) cables, the transceivers may be unsupported by the NIC firmware or the physical switch, resulting in a locked port or link initialization failure.

• Speed, Duplex, or FEC Mismatch: A failure in auto-negotiation or a configuration mismatch regarding speed, duplex, or Forward Error Correction (FEC) settings (common in 25G, 40G, and 100G environments) between the host NIC and the switchport.

• Link Flapping Protection: Severe packet errors (e.g., CRC errors) or rapid link state transitions may cause the physical switch port to enter an error-disabled state, or prompt the ESXi driver to stop traffic processing to maintain host stability.

Resolution

To further investigate this issue, refer to the following steps.

1. Verify Command-Line Status Connect to the ESXi host via SSH as user root and run the following command to check the exact Link and Admin status:

   esxcli network nic list

   
   If Link Status is Down but Admin Status is Up, the issue is physical or driver-related rather than an ESXi configuration setting.

2. Check for Link Flapping or "Failed Criteria" Codes 
   
   On the ESXi host, login as user root and examine the /var/run/log/vobd.log file for specific error codes that explain the failure:

   • Failed Criteria 128: Indicates the driver is reporting a link state down. Common causes include a faulty SFP+ module, cable, or switchport .
   • Micro-flapping: Look for logs indicating the link is unstable, which may cause ESXi to proactively take the NIC down.
   • Refer to the article for more details Network adapter (vmnic) is down or fails with a Failed Criteria Code in VMware vSphere ESXi

3. Perform a Port "Bounce" In cases of a race condition (e.g., host reboots before the physical switch is fully online), toggle the interface state:
   
   

   • ESXi side: esxcli network nic down -n vmnicX followed by esxcli network nic up -n vmnicX. Refer to the article for more details Setting the admin state up or down for a vmnic interface on ESXi
   • Switch side: Perform a shut and no shut on the specific physical switch port.

4. Hardware & Driver Validation
   
   

   • Verify if the SFP+ module is faulty; replacing the module often resolves "Link Status Down" errors when cables are otherwise fine.
   • On the ESXi host, loign as user root and check for firmware/driver mismatches by reviewing /var/run/log/vmkernel.log.