Incorrect configuration of MTU on vSwitch causes packets to drop

Products

• VMware vSphere ESXi

Issue/Introduction

• If Jumbo Frames are enabled on an ESX host and VMNIC Maximum Transmission Unit (MTU) is set at MAX (9000), dropped packets are observed in vmkernel log files.
• The vmkernel/vmkwarning log contains entries similar to:
  
  Apr 7 19:06:36 [host name] vmkernel: 2:01:49:21.635 cpu2:9826)WARNING: LinNet: LinNetTimeoutCheckOutputListHealth:4246: vmnic3: dropped 31164; Xmited 8405
  Apr 7 19:06:37 [host name] vmkernel: 2:01:49:22.635 cpu2:9883)WARNING: LinNet: LinNetTimeoutCheckOutputListHealth:4246: vmnic3: dropped 31080; Xmited 8212
  Apr 7 19:06:38 [host name] vmkernel: 2:01:49:23.634 cpu2:9827)WARNING: LinNet: LinNetTimeoutCheckOutputListHealth:4246: vmnic3: dropped 32329; Xmited 8422
  Apr 7 19:06:39 [host name] vmkernel: 2:01:49:24.635 cpu2:9827)WARNING: LinNet: LinNetTimeoutCheckOutputListHealth:4246: vmnic3: dropped 25647; Xmited 8358
  Apr 7 19:06:40 [host name] vmkernel: 2:01:49:25.635 cpu2:9860)WARNING: LinNet: LinNetTimeoutCheckOutputListHealth:4246: vmnic3: dropped 34205; Xmited 8341
• This issue causes performance degradation of the vSwitch.

Environment

VMware ESX Server 

Cause

Physical NIC Maximum MTU limitation based on the type of the NIC may vary. If the configured MTU value on a vSwitch is greater than the maximum MTU value supported by the up-link physical NIC card then dropped packets may occur.

Resolution

Configure the VMNIC MTU based on the manufacture supported MAXMTU. The sent packets must be within the MTU boundary. This is applicable for ESX 3.5 and later.

 

For more information on configuring Jumbo Frames, see iSCSI and Jumbo Frames configuration on ESX/ESXi (1007654).