Re: ESXi 5.1 - Disable Flow Control

First of all, as you probably noticed Flow Control on ESXi operates in autonegotiation mode by default, so it will only be active if the physical switch supports and enables it too. Some good information on the topic in general: http://rickardnobel.se/esxi-and-flow-control/

As to why you can't change the parameters, check if there is an updated driver for your NIC. Use ethtool -i vmnicX to see which driver version is currently running.

The ESXi HCL lists a few updated qlcnic async drivers:

http://www.vmware.com/resources/compatibility/detail.php?deviceCategory=io&productid=20862&deviceCategory=io&keyword=QMD8262-k&page=1&display_interval=10&sortColumn=Partner&sortOrder=Asc

The latest one being this qlnic 5.1.157:

https://my.vmware.com/web/vmware/details?downloadGroup=DT-ESXI5X-QLOGIC-QLCNIC-51157&productId=285

qlcnic-esx50-5.1.157-1089431.zip

Release Date: 2013-04-12

Build Number: 1089431

Update the driver and see if you can configure the pause feature then.