A. Jumbo frames are network frames that contain more than the standard 1500 bytes of payload, up to a maximum of 9000 bytes. (This size is sometimes called the Maximum Transmission Unit—MTU). For networks with a high degree of reliability, such as your internal LAN, jumbo frames' greater payload size can mean fewer network acknowledgements during data transfer, and thus better network performance for certain kinds of transfers, such as iSCSI storage traffic.
ESX v3.x included experimental support for jumbo frames on software iSCSI. In ESX v4.x, they're fully supported. Enable jumbo frames for a vSwitch by increasing its MTU inside the ESX Service Console:
esxcfg-vswitch -m <MTU> <vSwitch>
You can verify the switch has been given the new MTU value with the command:
You'll have to enable jumbo frames at each hop throughout your network, so it takes more work than just enabling them at your ESX server.