Broadcom physical NIC Tx throughput capped at 1 Gbps on 10G Cisco N9K switch

Products

• VMware vSphere ESXi

Issue/Introduction

Hardware: Broadcom BCM57412 NetXtreme-E 10Gb RDMA Ethernet Controller.

Connectivity: Physical network adapters (vmnics) are connected to Cisco Nexus 9000 (N9K) Series switches.

Switch Configuration: The N9K switch ports are manually configured to a fixed speed of 10Gbps.

Symptoms: When the physical network adapter (vmnic) speed is toggled from 1Gbps back to Auto, the adapter successfully negotiates at 10Gbps. However, subsequent performance testing via iPerf or packet capture reveals that the vmnic's transmit (TX) traffic is incorrectly throttled at 1Gbps, despite the negotiated link speed. The receive (RX) traffic remains unaffected and correctly reaches the expected 10Gbps line rate.

Observed Workaround: If the vmnic speed is manually transitioned from Auto to 1Gbps, and then explicitly set to 10Gbps (rather than Auto), the link negotiates at 10Gbps and the TX traffic rate correctly reaches near-line speeds (~10Gbps).

Environment

VMware vSphere ESXi

Resolution

1. If the physical switch port is configured to a specific speed (such as 1G, 10G, or 40G) instead of auto, you must manually set the vmnic speed to match the switch port setting rather than leaving it on auto.
2. Consult your server hardware and physical switch vendors to verify firmware compatibility and ensure the integrity of the physical layer, including cabling and SFP+ modules. We recommend engaging both vendors for collaborative troubleshooting.