Q. How do I configure flow control on ESX/ESXi servers?

A. VMware knowledgebase article 1013413 discusses the specifics for setting flow control on an ESX or ESXi server's physical NICs. Flow control is a network configuration that manages the rate of data communication between two different network interfaces. It's necessary to prevent the outbound data from a fast sender from overwhelming the interface on a slow receiver.

Flow control is typically configured on network equipment, but it can be configured on hosts and storage devices as well. Configuring flow control on ESX is different depending on the type of network card in the server. For example, an ESX server with four Intel NICs will use the following command to disable flow control:

esxcfg-module -s FlowControl=0,0,0,0 e1000

In the command above, the four zeros separated by commas correspond to the four physical adapters in the server. Options 1, 2, and 3 are also possible, with 1 corresponding to receive only, 2 to transmit only, and 3 to receive and transmit.

Broadcom cards use a different command syntax for configuration, with a command similar to the following needing to be added to the ESX host's rc.local file:

ethtool -A vmnic# autoneg on rx off tx off

The knowledgebase article noted above provides more detail about flow control options. Be aware that network problems, such as excessive pause frames being transmitted across the network, can occur when flow control isn't configured properly,.

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