Recommended sequence for modifying the MTU (Maximum Transmission Unit) on a network

Products

• VMware NSX
• VMware vSphere ESXi

Issue/Introduction

The article Risks and potential impacts of changing the MTU on a virtual switch explains the effects, risks and impact of changing the MTU in an ESXi and/or NSX environment.

These articles explain where you can set the MTU on a vSphere Standard Switch, vSphere Distributed Switch, and in NSX.

During the process of changing the MTU across a network, the ideal sequence for completing this change that reduces risk of network traffic interruption will depend on whether the MTU is being increased or decreased.

Resolution

When the MTU is being increased, the MTU should be changed first on the physical uplink switch and other network devices along the path prior to making the change on ESXi virtual switches or NSX segments. Network clients including VMkernels, tunnel endpoints and VMs should be changed last. This will allow the network infrastructure to accommodate the larger packets prior to them being sent.

When the MTU is being decreased, the MTU should be changed first on client interfaces such as VMkernels and tunnel endpoints where applicable. The MTU can then be changed on virtual switches and segments, and then on the physical network infrastructure. This will decrease the size of the packets being transmitted prior to applying lower limits on network links which would have dropped the larger packets.

Additional Information

Network administrators should be prepared for packet drops or interrupted connections during an MTU change even when the proper sequence is followed.

Many factors are involved in determining the optimal MTU for a network. Review the article Understanding MTU and MSS Calculations Across Complex, Layered Infrastructures to learn more about protocols that add overhead, the type of workloads that benefit from larger packets and the performance impact that larger packets can have on your network.