Jumbo Frames - MTU 9000 FreeNAS v8.0.1 RC2

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Sep 26, 2011
Ok, looking over and see many articles on this from old version how-to to some more current. Here is how to do it for those that don't want to read a book and test it.

First, no need to destroy any LAGG's or anything of the sort like I read.

1. Open up your GUI console and under "Network" -> "Link Aggregations" -> "LACP" (or whatever you created) -> "lagg0" (or whatever you named it) -> "em0" & "em1" (both nic cards) open them up one at a time.

2. In EACH of the NIC (em0 & em1) 4th option, it will say "Options". In there you want to type "mtu 9000 up"


First, make sure that you have the SSH (with Login as Root with password option set) turned on.

Use a program like PuTTy and log into your shell and from the prompt type "ifconfig"

Expand the window to see all your nic and lagg information and both nic's and lagg will say mtu 9000. (Don't forget to reboot after you set the Options in the GUI or it will not work).

There you go. So, without that option, default is MTU 1500... remove that option and reboot to set it back to MTU 1500. Only the NIC have to have the options set as the LAGG inherits this setting.

Hope this helps.


Jun 29, 2011
I was not able to get the configs to see an MTU of 9000. Nothing other than 1500 actually.

FreeNAS 8.0.1 Release
HP DL320s, Broadcom NICs


Sep 26, 2011
First, I have to say that not every NIC can do JUMBO FRAMES (MTU 9000 example). Please look up and read about MTU and check your hardware to verify it will do the MTU you are trying to configure it to do. It doesn't have to be set to 9000. It can be set to whatever the LIMIT of your NIC may be. Make sure you set your SWITCH and SERVERS to that same MTU.

Please look up your computers NIC that is installed as there are different version with the HP DL320's.
Example search resulted in the following post on a single version.

bge(4) supports jumbo frame on controllers that provide extended RX
BD capability. Even though your controller BCM5714 supports very
limited set of jumbo frame it has severe hardware limitation which
make it hard to coexist existing controllers which have different
and efficient jumbo frame scheme. I think other OSes also do not
support jumbo frame on BCM5714 family.
Because there is no easy way to support this exotic jumbo frame
scheme without sacrificing a lot of sane controllers I have no plan
to implement the limited jumbo frame scheme on BCM5704. I also
think you wouldn't get much benefit from this kind of jumbo frame
support due to the limitation of hardware.


Look up your controller and verify it limits and capabilities. Hope this helps some.

NOTE: What does MTU 9000 do for you? Will it increase the speed of my transfer? These answers are different for OLDER COMPUTERS ONLY... New systems with FAST CPUs may see little or NO improvements as they were fast enough (where NIC will assist in some cases) to process NETWORK PACKET SIZES of only 1500 (standard) MTU.

When you have a BIG file and you try to transfer it, it will take that huge file and break it down into block sizes of say 1500. Many more blocks will be required to transfer a file in them chunks over say block sizes of 9000. Also, at 1500, you will have to process (with the CPU) more often. Both ends (SAN and Server) will have to process.

Some NIC's have advanced technology (iSCSI offload & TOE [TCP off-load engine]) which can help - OS SETUP REQUIRED

So, there you have it in a nutshell the basics of this small world.

If you are using ESX/ESXi or even Windows Hyper-V, check out the CPU performances when you transfer a mass file to-from SAN/NFS. Take note of the CPU jump and then try it with a higher MTU.
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