On Jumbo frames: Short note about iSCSI

It’s been a bit of time since I’ve had to deploy a new iSCSI target at work, this time it was a QNAP box that will serve as secondary backup target for some Backup Exec.

As you have hopefully read elsewhere if using iSCSI, be it over 1GE or 10GE: Use Jumbo or Giant frames for better efficiency. If your target has the capacity to fill the pipe, it will do better if you use Jumbo frames.

On the HP-3Com 2928* I’ve been using for iSCSI, I never had to bother anything about Jumbo Frames, things just worked after enabling Jumbo Frames on both target- an initiator-side NICs. As I’ve had to learn the hard way: Jumbo Frames have to be enabled on a couple of switch vendors for at least I know now that this applies to both Netgear (Smart & fully) managed as well as Cisco gear. Without this you may be able to connect to the Target Portal, but at least the Windows Initiator bugged to hell, even hanged without being able to disconnect from a target – we had to kill the preferred targets and reboot the initiator side to get rid of the erroneous iSCSI session. After Jumbo Frames were being allowed on the switch, things started flowing without hickups.

For Netgear managed switches this is done on per port basis, according to manuals, this sets the maximum allowed MTU – without
knowing it better I allowed the maximum possible MTU the switches on 8.0.3.x Firmware allows:

enable
configure
interface 1/0/22
mtu 9216
exit
copy system:running-config nvram:startup-config

Of your course you can also specify interface range xx/xx/xx yy/yy/yy and configure all ports you need to allow Jumbo Frames.
While a exported Text configuration on their smart switches shows the same statements you have to do this via the Web UI configuration, the result behaves the same as with the fully managed boxes.

For Cisco’s popular 3560, 3750 series and others apparently you set maximum MTU globally, here is the link to the IOS comands for MTU mangling.

* After the 3Com aquisition by HP, the 2900 series have been relabled twice, first as V1910 later 1910.
Apparently HP seems to care a bit about this line of Switches as they’ve received a fully IPv6 manageable Firmware in 2012  (even for our pre-HP era 3Com units) and stil receive regular bugfixes as of writing.

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January 27, 2013

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