iperf3 throughput benchmark test on a VLAN backed portgroup fails between different VLANs

Products

• VMware vSphere ESXi

Issue/Introduction

• Within the same subnet, the throughput is high but inter-subnet traffic fails to maintain stable data transfer.
• iperf3 transfer rate is stuck at 0 for inter VLAN test
• iperf3 is ran with MTU parameter set to 9000 

Environment

VMware vSphere ESXi

Cause

The gateway for the virtual machines is configured on the physical network and is set to default MTU of 1500 bytes.

Packet capture shows the packets were retransmitted when packet length reached 9000 bytes. While the end hosts were configured for jumbo frames, the Switch Virtual Interfaces (SVIs) are set to the default MTU of 1500 causing the gateway to drop bigger frames.

Resolution

Increase the MTU size on the gateway L3 interfaces to support jumbo frames.

Additional Information

In high-throughput environments utilizing Jumbo Frames (9000+ bytes), every hop in the Layer 3 path—including all gateways and SVIs—must be consistently configured to support the larger frame size. Failure to align MTU settings across the entire path results in throughput degradation and packet loss during routing.