Support for Jumbo Frames is one of the major new features for ESX 3.5. Especially for the people who are using an iSCSI SAN configuring jumbo frames could be very beneficial. Instead of having an MTU(maximum size of transmitted packet) of 1500 an MTU of 9000 would be possible. That would cut out a lot of the iSCSI overhead. But are jumbo frames supported for 3.5? Answer: Yes and no.
Jumbo frames for a vSwitch are supported. Most people would want to enable this for the iSCSI initiator, but:
Jumbo Frames allow ESX Server 3 to send larger frames out onto the physical network. The network must support Jumbo Frames end‐to‐end for Jumbo Frames to be effective. Jumbo Frames up to 9kB (9000 Bytes) are supported. iSCSI with Jumbo Frames is not supported!
In other words, your vSwitch and VMkernel support Jumbo Frames but the iSCSI initiator doesn’t. I don’t really understand why VMware did this, but it probably has got something to do with the release date of 3.5 and proper testing this new functionality. If you want to take the the risk here’s how you enable jumbo frames on your vSwitch or VMkernel:
- VMkernel command: esxcfg-vmknic -a -i <ip-address vmkernel> -n <netmask vmkernel> -m 9000 <portgroupname>
- vSwitch command: esxcfg-vswitch -m 9000 <vSwitchx>
for more info check the server configuration guide.