Jumbo frames observed on physical switch ports despite 1500 MTU configuration in ESXi

Products

• VMware vSphere ESXi

Issue/Introduction

Physical network switches may report jumbo frames or frames exceeding 1500 bytes on interfaces connected to ESXi 8.x hosts, even when the Virtual Standard Switch (VSS) or Virtual Distributed Switch (VDS) and VMkernel ports are explicitly configured with a Standard MTU of 1500. This observation often leads to concerns regarding MTU mismatches.

Symptoms:

• Example switch port statistics show packets larger than 1500 bytes.
  
  742290476 unicast packets
   18722 multicast packets
   405 broadcast packets
   742309603 input packets
   505356804748 bytes
   288517583 jumbo packets

• ESXi host networking is configured for 1500 MTU.

• No connectivity issues are necessarily present, but "Giant" frames are logged at the physical layer.

Environment

 

• VMware vSphere ESXi 8.x

 

Cause

This is expected behavior and is not a bug or misconfiguration. ESXi utilizes TCP Segmentation Offload (TSO) and Large Receive Offload (LRO) by default. These features allow the VMkernel to pass large buffers (up to 64KB) to the physical network adapter (NIC). The NIC then handles the segmentation of these large buffers into standard MTU-sized frames before they leave the wire. Conversely, the NIC can aggregate incoming standard frames into larger buffers before passing them to the ESXi stack.

Depending on where the physical switch captures or monitors the traffic, it may register these large data transfers as jumbo frames before final hardware segmentation or during specific offload processing.

Resolution

No resolution or configuration change is required as this is a functional optimization of the ESXi networking stack.

If the reporting of jumbo frames on the physical switch causes operational alerts that must be silenced, you can disable TSO/LRO, though this is not recommended as it increases CPU overhead for network processing.

To verify TSO settings on an ESXi host:

1. Log in to the ESXi shell or SSH as root.

2. Run the following command to check the status of TSO: esxcli system settings advanced list -o /Net/TcpipV4SwaTso

Additional Information

For further technical details on how TSO/LRO impacts frame size perception, refer to:

Understanding TCP Segmentation Offload (TSO) and Large Receive Offload (LRO) in the vSphere environment

Troubleshooting and understanding physical NIC receive or transmit dropped, missed and error counters in ESXi