I have been working on VMware vSphere v4 for the last few days and have found that enabling jumbo frames support is not as intuitive as the admin guide makes it sound. The process goes something like this.

  1. Enable jumbo frame support on iSCSI SAN (MSA 2012i)
  2. Enable jumbo frame support on switch (a Linksys business class in this case)

Both of these are very straight forward and simple to do, but here is where it gets a little tricky. vSphere v4 jumbo frame support for the iSCSI network must be enabled with the CLI. Two things must be done. You must enable jumbo frames on the vSwitch and then able it on the VMkernel port/ports. The command to enable JF enable on the vSwitch looks like this: [more]

esxcfg-vswitch -m <MTU> <vSwitch>

In my case, I wanted a 9000 MTU (9KB) for vSwitch1, so it was ‘esxcfg-vswitch -m 9000 vSwitch1’ . All good so far. Now, I needed to JF enable the VMkernel port. Admin guide says this is the command

esxcfg-vmknic -a -I <ip address> -n <netmask> -m <MTU> <port group name>

So my command looked something like this esxcfg-vmknic -a -I -n -m 9000 “iSCSI VMkernel”

When I ran this command, it said that this port group didn’t exist. Exactly…that is why I am running this command…to create the VMkernel port and associated port group. After MUCH troubleshooting, I finally found that the following sequence WILL work. It must have something to do with the way the port groups and their associated identifiers are created.

  1. Create the VMkernel port in the vi client (GUI)
  2. Remove the VMkernel port from the CLI with this command esxcfg-vmknic –d “iSCSI VMkernl”
  3. Recreate the same VMkernel you just deleted with JF enabled esxcfg-vmknic -a -I -n -m 9000 “iSCSI VMkernel”