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Linux Bridge module cannot handle packets larger than the maximum transmission unit

Details

Enhanced vmxnet adapters deliver TCP Segmentation Offload (TSO) packets originating from a virtual machine on the same vSwitch directly to a Linux guest without breaking the packets apart. The packets appear to be Large Receive Offload (LRO) packets and, in most cases, the guest network stack can handle them without any problems. This behavior significantly improves TCP traffic performance between virtual machines on the same vSwitch. However, the Bridge module in Linux cannot handle packets larger than the maximum transmission unit (MTU).

Solution

To work around this problem, downgrade the Enhanced vmxnet adapter to Flexible.

For virtual machines with virtual hardware version 7 and later, ESX/ESXi automatically upgrades Flexible virtual NICs to Enhanced vmxnet virtual NICs. To prevent ESX/ESXi from performing the upgrade, add the following line to the vmx file: vmxnet.alwaysEnhanced=FALSE

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