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No jumbo frames (MTU 9000) with new Core i5 iMac?

Discovered today that my day-old iMac Core i5 is missing the option to set MTU to 9000 (to enable jumbo frames). When in my network settings I have only options "Standard (1500)" and "Custom" and when I choose "Custom" the entry field shows allowed values of 72 - 1500. If I try setting to 9000 then it gets set to 1500. I used to have a "Jumbo (9000)" option with my previous 24" iMac (C2D) which I sold a couple of weeks ago. So I know my network hardware supports it, and having it set to Jumbo is very helpful in getting increased network throughput to my NAS.

To confirm, yes -- I am trying to set this via Ethernet settings and not AirPort. This issue occurs in "Automatic" network location and in any new ones I set up (such as "Home"). Also applies to any new users I try to create as a troubleshooting aid. I have bounced the gigabit ethernet switch I am connected to, just in case... I have not yet tried reinstalling 10.6.2 from the disc that came with the machine since I just brought it home from the Apple Store today!

The other network options on the "Ethernet" tab are:

Configure: Manually
Speed: 1000baseT
Duplex: full-duplex, flow-control
MTU: Standard (1500) or Custom (my only 2 choices)

Can anyone confirm that MTU 9000 (Jumbo Frames) is or is not enable-able on your Core i5 or i7 iMac?

THANKS!

Message was edited by: Pinthea

Message was edited by: Pinthea

iMac 27" Core i5, Mac OS X (10.6.2)

Posted on Nov 16, 2009 6:40 PM

Reply
90 replies

Nov 17, 2009 12:45 PM in response to Pinthea

I would ask Apple, you have 90 days support may as well use it. Other than that have you tried using the ifconfig command in terminal?

from the manual page:-
mtu n Set the maximum transmission unit of the interface to n, default
is interface specific. The MTU is used to limit the size of
packets that are transmitted on an interface. Not all interfaces
support setting the MTU, and some interfaces have range restric-
tions.

Nov 17, 2009 2:42 PM in response to rack0 tack0

I did call support -- in fact I have an open ticket right now and am working with them to try and figure it out. Unfortunately they're not that familiar with MTU or jumbo frames and don't have a 27" Core i5 unit to test it out on, so I'm sort of out there hanging in the wind...

I'll try the ifconfig command line thing to see what I get, but as it points out "some interfaces have range restrictions" and my very issue is being limited to a range of 72 to 1500. Maybe it's a UI bug and it can be set otherwise via command line.

Nov 17, 2009 2:57 PM in response to Pinthea

I have just tried it on my iMac 7,1 machine. It lets the custom be set from 72 to 9000 and checking in ifconfig confirms that its 9000. However the UI keeps displaying Standard 1500, not sure what is happening there, presume that is a UI bug.
I normally run with an MTU 1492 and if I set it to this the UI does display Custom 1492.

The ifconfig always seems to show the value that I set it to even when set from the UI.

I am running 10.6.2 , I presume if its a new machine yours is also 10.6.x

Hope you get the conformation that you can run 9000

regards

Nov 17, 2009 3:53 PM in response to rack0 tack0

OK, I've tried to use ifconfig from within terminal as follows:

ifconfig en0

The results show that an mtu value of 1500 is what's set currently.

So then I try this:

ifconfig en0 mtu 1500

(just to see if it's possible to set the value via ifconfig)

response:

ifconfig: ioctl (set mtu): Operation not permitted

And in testing ifconfig further, ANY mtu value I supply, whether it be in the range of 72 - 1500 (which is what the UI says is allowed) or whether it be 9000 (as I want it to be), I get the same error.

If I use the system preferences/network advanced properties to use the UI to change MTU to another value within the displayed allowed range of 72-1500, say, 1492, then after applying the change I do see that ifconfig reports that MTU in its results when I query the attributes of en0. So the UI IS able to set the MTU value, as long as it's within the range of 72-1500, whereas I cannot set the MTU to a value within that range manually using ifconfig.

When I go back to network properties and look at the MTU field, I do see that it is displaying the correct value of 1492 if I set it to 1492 via the UI.

If I try to use the UI to set the MTU to a value higher than 1500, it ends up setting it to 1500 (which is confirmed by ifconfig).

I've tried resetting the SMC, resetting the PRAM, and even going into SAFE mode.

The ethernet chipset (Broadcom 5764) from what I can tell IS capable of jumbo frames; it would seem that the driver or ethernet support for it on my machine (and potentially other Core i5's) is lacking support for Jumbo frames or any MTU value > 1500.

I hope this is something Apple will be able to fix, I'll be sure to report this info to the Apple Care support guy I'm working with...

Nov 17, 2009 4:37 PM in response to Scorpie

Rookie mistake, my bad!

You're right, sudo is what was required. Thanks...

I can use "sudo ifconfig en0 mtu xxxx" to set any mtu value between 72 and 1500. If I try any value > 1500, the result is always:

ifconfig: ioctl (set mtu): Invalid argument

So I am apparently lacking support for any MTU > 1500. No jumbo frames.

Nov 17, 2009 6:24 PM in response to Scorpie

Disappointingly there was no joy from the Apple Care specialist I was working with. We tried almost everything. After I got off the phone with him I even tried a fresh install of Snow Leopard from the 10.6.2 DVD which came with it. As soon as I was able to login for the first time I went straight to the network settings where I was able to verify that from a fresh, clean install there is NO support for Jumbo Frames or any MTU > 1500.

Disappointingly and more alarmingly than that, I tried to perform the Apple Hardware Test (AHT) by booting with the Applications Install DVD and pressing and holding D at boot, and once the AHT disc loaded the screen flashed and the system powered off.

And then it happened again when I tried it again.

And then it happened yet again when I tried it again.

But it boots fine and seems to run fine except for its complete inability to run the AHT utility.

So I've got a Genius Bar appt for Thursday afternoon to have them look at it.

And disappointingly, I'm not impressed with my experience with this new computer. Maybe this is the price I pay for being an early adopter. But frankly I expect to at least be able to run the diagnostics utility!!!

Message was edited by: Pinthea

Nov 17, 2009 7:32 PM in response to Pinthea

There was only one model of the 24-inch iMac that ever used the Nvidia 9400m graphics and that was the $1499 US priced Early 2009 model.

All iMacs since 2006 (Intel) has had support for jumbo frames. On Apple's current online web site only the Mac Pro out of all current Macs has Jumbo frames specifically stated in the Tech Specs.

I am going to check the developers section for Hardware specs and see if they have them up yet. This could answer this question very quickly.

Message was edited by: StephenCCH

Nov 17, 2009 8:18 PM in response to StephenCCH

I can't imagine that the Broadcom chip they use isn't jumbo frame capable, it must be an issue with the driver or ethernet chipset support in the kernel.

Hopefully they can address it with a future software udpate...

I'm curious to examine the new C2D iMacs (both the 21.5" and 27") to see if any of them have Jumbo Frames as an option. I know it's not necessarily apples-to-apples since there are different chipsets and motherboards at play, but if Jumbo is an option there then there can't possibly be an intent for Apple to disable it on the i5's...

Sigh...

Nov 17, 2009 8:54 PM in response to Pinthea

Wow, this Core i5 just gets more and more disappointing.

After reinstalling Snow Leopard 10.6.2 from the install DVD that came with the system this afternoon, I neglected to set the power management settings to disable sleep mode (which I like to do on a new setup).

Just now I discovered that the system went to sleep, as it was configured to do, and it woke up OK except for the fact that the ethernet port would not work after it resumed from sleep mode. Couldn't navigate to any website nor to any of the resources on my local network. Tried resetting Safari, didn't work.

I was forced to do a reboot in order to get network access again.

What a huge disappointment this machine is... I think taking it back to the Apple Store and paying the 10% restocking fee is just about tolerable just to have this thing off my desk.

No jumbo frames (MTU 9000) with new Core i5 iMac?

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