Enabling jumbo frames in VMware ESX 4

This topic has been around for awhile but, since we provide EqualLogic iSCSI SANs as part of our product offering, I thought I would write about it.  Jumbo frames are units that contain 9,000 bytes of data instead of the usual 1500, or an MTU of 9000 instead of 1500.  When using the ESX software iSCSI initiator, a network adapter becomes the transport device for your iSCSI traffic.  When creating a virtual switch using the vSphere client, you cannot change the default MTU value of 1500, nor can you even view the values.  These changes must be made through the Service Console.

First, you must create the vSwitch and change the MTU to 9000.  For this example, vSwitch9 will be the name you would replace with your own.

esxcfg-vswitch -a vSwitch9

Then, set the MTU of the vSwitch.

esxcfg-vswitch -m 9000 vSwitch9

esxcfg-vswitch -l will list all the vSwitches so you can confirm your changes and settings.

Switch Name    Num Ports   Used Ports  Configured Ports  MTU     Uplinks
vSwitch9             64                  1            64                 9000

As you can see, the Uplinks field is blank, so you must add a physical NIC to the virtual switch, but first we'll do a few other things.  iSCSI access is controlled by a VMkernel interface and assigned to a port group on the vSwitch.

To create the portgroup:

esxcfg-vswitch -A <port group name> vSwitch9

Then create the VMkernel interface:

esxcfg-vmknic -a -i <your iSCSI initiator IP address> -n <subnet mask> -m 9000 <port group name from the above step>

If you named the port group "iSCSI" and your IP is 172.16.0.1/24, the command would like this:

esxcfg-vmknic -a -i 172.16.0.1 -n 255.255.255.0 -m 9000 iSCSI

esxcfg-vmknic -l will confirm your settings.

The last step is to add a physical NIC to the vSwitch.  This can be done via the GUI, but we are having fun here, so let's do it with commands.

esxcfg-vswitch -L <physical NIC, such as vmnic1> vSwitch9

You're done!  You can now refresh the Networking portion of the Configuration tab in the vSphere Client to see the new virtual switch components.  Keep in mind that you must enable jumbo frames on your physical switch for this to work.