MTU Sizes Jumbo Frames

Older firmware revisions had a lower maximum mtu size. In the newer revisions they increased the mtu limit.

It should let you set it to 9216. Can you post up the output of the command ,console>show version. This will let us check what revision the switch is on, which image the revision is installed on, and if that image is active.

You could also run the command [tag:iscsi] enable

When iSCSI is enabled, the following actions occur:

• The MTU on all ports and port-channels is set to 9216 (jumbo frames are

enabled).

• Flow control is globally enabled, if it is not already enabled.

• iSCSI LLDP monitoring starts to automatically detect Dell EqualLogic

arrays.

Sounds like the switch is on an older firmware revision. Getting the firmware up to date should enable the larger MTU setting.

http://dell.to/1vHRJit

I don't understand your response of:

1. "Getting the firmware up to date should enable the larger MTU setting."

9216 MTU is not even in the Ethernet Jumbo Frame standard, so why would it support larger?

2. I'm on the latest firmware A36, so no changes have been made to this release.

Daniel -

Would you mind reviewing my initial questions as I don't believe you fully understand them.  See below on my firmware.

console#show ver

Image Descriptions

image1 : default image

image2 :

Images currently available on Flash

--------------------------------------------------------------------

unit      image1      image2     current-active        next-active

--------------------------------------------------------------------

   1    3.3.11.2     3.3.4.1             image1             image1

Thanks for the output from the show command, it looks good.

With MTU size you need to leave enough room for the headers to be added. IPv4 =20, TCP =20, icsi can vary, etc. So MTU 9216 is not required, but is used to ensure there is enough space.

However with the firmware the switch is on should allow the mtu to be set to 9216. I just connected to one we have in the lab and it allowed me to set a port to 9216.

If you enter an interface and enter this command # mtu ?, what does it show as possible options?

I apologize, I was reading that incorrectly. With ip mtu on the VLAN interface, this value includes the 802.1q tag, but does not include the Ethernet header and CRC trailer. So, the maximum Ethernet frame size, including the Ethernet header/trailer, is 9198 + 18 = 9216 bytes.

The physical interfaces includes everything, meaning you can set it to 9216.

OK so back to question

3.Do I have to apply a MTU size to a VLAN?  My intent is not all VLANs on the physical port need jumbo frames.

ip mtu setting is used when a packet is sent on an interface. When applied to a VLAN interface, the ip mtu command will be followed when the VLAN sends the packet to another VLAN. So if your jumbo frames are all staying in the same VLAN, then you shouldn't need to worry about it. However if the jumbo frames will frequently be moving from VLAN to VLAN, i would set the ip mtu to 9198.

"i would set the ip mtu to 9198."

To be clear, on a host with a MTU of 9000 you're saying on the VLAN interface that would equal 9198 on your switch? That is odd but let me know if that is what you're saying.

yes, on a VLAN interface you would set it to 9198.

On the physical interface you would set it to 9216.

Hi

I have a question please. i dont have acces to my actual fabric switch 2748 and i am waiting for a powerconenct 7048 switch that i need to set it as a fabric switch. on my hosts and SAN eqaullogic Ps6100 i have the MTU set at 9000. what Mtu should i give to my Dell 7048 ? 9000 or 9219 ? and whay ?

many thanks

Hello,

here you find the best-practice configuration for a lot of switches and different Dell-SANs.

[View:~/techcenter/storage/w/wiki/4250.switch-configuration-guides-for-ps-series-or-sc-series-sans:550:0]

Among them is the PC7048: http://en.community.dell.com/techcenter/extras/m/white_papers/20438529/download.aspx

regards

Henning