Wednesday, July 1, 2009

PowerShell - vSphere VMKernel NIC MTU

I came across a problem with VI4 (vSphere) on ESXi, where I was unable to create a vmkernel NIC for iSCSI with the script to set a custom MTU of 9000. This post provides a method using the GUI/perl script, and a PowerShell vSphere PowerCLI script to modify an existing VMKernel NIC.

It seems there is a bug in the perl script doesn't allow you to create a VMKernel NIC, however it does work if you create one through the GUI, and then use the script to delete and re-create the virtual NIC.

This was using a standalone ESXi server with a normal vSwitch - it wasn't managed by VC and it wasn't a DVS.

The GUI/PL script method:

  1. Create a VMKernel NIC in an existing vSwitch through the VI Client, with a port group called iSCSI1 in this example.
  2. Using the CLI, run the following commands, the first to delete the existing NIC, and then another to re-create the same NIC, but with an increased MTU: -d iSCSI1 -a -i x.x.x.x -n x.x.x.x -m 9000 iSCSI1
  3. Run ' –l' to list the VMK NICs, which should show the adjusted MTU

I wasn't particularly happy with this method, so I looked at using the vSphere PowerCLI to set the MTU. For whatever reason, VMware chose not to expose the MTU property, which leaves only the SDK.

Luckily the PowerCLI makes it much easier to use the .Net SDK than previously possible with the Get-View cmdlet, so the script below is a combination of standard PowerCLI with a hook into the vNIC collection and the UpdateVirtualNic method to change a virtual NIC spec.

param (

    $MTU = 9000, 
    $nicName = vmk1, 
    $vcServer = "vcServer", 
    $vmServerName = "esx01"

# Description:
#  Update the MTU of a vmkernel NIC used for iSCSI
# Limitations:
#  DVS untested
# Assumptions, this script works on the assumption that:
#  The caller provides credentials with permissions to change the specified NIC
# Arguments:
#  MTU, The new MTU size, eg. 9000
#  nicName, The vmkernel NIC, eg. vmk1.  Note that this differs from the port group (eg. iSCSI1, with a NIC name of vmk1)
#  vcServer, the VC instance to connect to, eg. vcServer
#  vmServerName, The server name controlled by the VC instance (or direct), eg. esx01
# Usage:
#  PowerShell . .\UpdateVMKMTU.ps1 -MTU 9000 -nicName 'vmk1' -vcServer 'vcServer' -vmServerName 'esx01'
# References:
# Changes:
#  26/06/2009, Wayne Martin, initial version

#$ErrorActionPreference = "Continue"

Connect-VIServer -server $vcServer        # connect to VC
$hostSystem = get-view -ViewType HostSystem -Filter @{"Name" = $vmServerName}   # Find the .Net view of the specified host
$hostConfigManager = $hostSystem.get_ConfigManager()      # Get the config manager
$hostNetworkSystem = $hostConfigManager.get_NetworkSystem()     # Find the MOR of the host network system
$netSystem = Get-View $hostNetworkSystem       # Get the object from the reference for the update method later

$hostconfig = $hostSystem.Config        # Get the current host config
$hostNetwork = $hostconfig.Network        # Get the current network host config
$hostvNIC = $hostNetwork.vNic         # Get the virtual NICs

$nicBeingUpdated = $null
foreach ($hostVirtualNIC in $hostvNIC) {       # For each virtual NIC
    if ($hostVirtualNIC.Device -eq $nicName) {        # Is this the device specified?
        $nicBeingUpdated = $hostVirtualNIC       # Yes, copy the object

if ($nicBeingUpdated) {          # Was something found?
    $nicSpec = $nicBeingUpdated.Spec        # Yes, get the current spec
    $currentMTU = $nicSpec.MTU         # Get the current MTU from the spec

    if ($currentMTU -ne $MTU) {         # Is the MTU different?
        $nicSpec.set_Mtu($MTU)         # Yes, update the MTU on the copy of the spec of the existing NIC
        $netSystem.UpdateVirtualNic($nicName, $nicSpec)             # Call the update method from the netsystem, to update the NIC with the modified device spec

        write-output "MTU for $nicName updated from $currentMTU to $MTU"
        (get-vmhostnetwork).VirtualNic        # Output updated information
    } else {
        write-output "MTU for $nicName already set to $MTU, no changes made"

} else {

    write-output "NIC $nicName not found, no changes made"


Wayne's World of IT (WWoIT), Copyright 2009 Wayne Martin. 


Unknown said...

Thanks for that scirpt. It saved me a bunch of time.

When I cut and pasted your script, I did run into one bug. Line 64:

(get-vmhostnetwork).VirtualNic # Output updated information

had to be changed to:

(get-vmhostnetwork -VMhost $vmServerName).VirtualNic # Output updated information

The error from a straight cut and past from your published scirpt is shown below.

MTU for vmk2 updated from 1500 to 9000
Get-VMHostNetwork : Value cannot be found for the mandatory parameter VMHost
At C:\Program Files\VMware\Infrastructure\vSphere PowerCLI\Scripts\UpdateVMKMTU.ps1:64 char:27

Anders Olsson said...

Hi. Thanks for the tip. This is a very handy script, since the built-in Perl scripts don't allow changing the MTU without deleting and re-creating the entire vmknic. This is particularly difficult when changing MTU for the VMkernel Management port, since it's the one you are actually connecting to when executing the script.

However, the script on the page doesn't seem to work. The output says "MTU for vmk0 updated from 1500 to 9000", but the setting never actually sticks. Rebooting the ESX server didn't help.

Any ideas?

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About Me

I’ve worked in IT for over 20 years, and I know just about enough to realise that I don’t know very much.