Link flapping on Broadcom BCM57414 dual-port adapters with un-cabled SFP (VMware ESXi 8.0)

Products

• VMware vSphere ESX 8.x

Issue/Introduction

Broadcom BCM57414 dual-port 25G OCP adapters on ESXi 8.0.x hosts experience continuous link flapping on active ports when the secondary port contains an un-cabled SFP transceiver.

Symptoms:

• Active ports (e.g., vmnic3, vmnic5) flap every few seconds.
• Secondary ports (e.g., vmnic2, vmnic4) are physically disconnected but contain an SFP module.
• vmkernel.log shows the bnxtnet driver initializing and immediately dropping the link.
  
  vmkernel.log shows the following sequence: 
  
  bnxtnet: bnxtnet_update_phy:3816: [vmnic3] link_info->auto_mode: 4, phy_cfg.flags: 0x00000029 - (AUTO)
bnxtnet: bnxtnet_display_link:2985: [vmnic3] NIC Link is down
bnxtnet: bnxtnet_uplink_quiesce_dev:2606: [vmnic3] Quiesce dev 0x4528003be000
  
  
• vobd.log confirms vob.net.vmnic.linkstate.down and up transitions.
  
  vobd.log confirms linkstate transitions: 
  
  [vob.net.vmnic.linkstate.down] vmnic vmnic3 linkstate down
[vob.net.vmnic.linkstate.up] vmnic vmnic3 linkstate up

Environment

• Host: VMware ESXi 8.0.x
• Hardware: Broadcom BCM57414 dual-port 25G OCP adapters
• Driver: bnxtnet
• Switch: Cisco Nexus N9K (or other 25G switches)

Cause

The two ports on this network adapter share internal clock/timing hardware. When an unused port (Port 1) has an optical module (SFP) inserted but is not connected to a switch, the adapter continuously attempts to auto-detect a link speed, a process that never succeeds since there is no far-end device.

This repeated detection cycle periodically competes for the shared internal clock resource that the active port (Port 2) relies on to maintain its connection. When that happens, Port 2's link is briefly interrupted before recovering, and the cycle repeats, producing the periodic link flapping observed.

This is categorized as working as designed based on the adapter's shared hardware architecture. This timing issue only occurs when no link is established but modules are present. If a cable is inserted into Port 1 after Port 2 has already established a link, Port 2's link status will not be affected.

Resolution

Follow either method below to stabilize the link:

• Physical Removal (Recommended): Remove any un-cabled SFP transceivers from the adapter slots. This stops the background initialization calls.
• Manual Override: If the transceiver must remain in the slot, manually hardcode the unused port to a fixed speed (e.g., 25Gbps) in the UEFI or ESXi settings. This halts the dynamic auto-negotiation discovery process.

Additional Information

Please also refer Broadcom KB article Broadcom BCM57414 Adapter FEC CL74 Setting Confirmation (VMware ESXi 8.0)