VMware Tanzu Silk CNI default MTU of 1454 exceeds Azure MTU limit

Products

• VMware Tanzu Application Service for VMs

Issue/Introduction

Customers using VMware Tanzu on Microsoft Azure may find that traffic is intermittently dropped due to exceeding the default Maximum Transmission Unit (MTU) of Azure. Azure by default permits an MTU size of 1500 bytes, however Azure has additional overhead in TCP/IP headers, which may require additional space.

By default the Silk CNI for container to container networking will have an MTU value of 1454 bytes. Due to the additional overhead required inside Azure, additional headers or TCP/IP data could result in packets exceeding the 1500 byte MTU default value in Azure.

Environment

Product Version: 2.10

Resolution

Customers should reduce the MTU size in Tanzu by Selecting the Tanzu Application Service (TAS) Tile (TAS) > Networking > Silk > Maximum Transmission Unit. Set this value to 1400 bytes. This will leave sufficient room for any overhead from Azure. 

Additional Information on Azure and MTU can be found Azure and VM MTU. While azure by default will permit MTUs up to 1500 bytes, it will attempt to fragment packets above 1400 bytes.

Additional Information

Additional Information on Azure and MTU can be found here. While azure by default will permit MTUs up to 1500 bytes it will attempt to fragment packets above 1400 bytes.