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The switch at hand is a Dell PowerConnect 2716 which does support both VLANs and Jumbo Frames but, as far as I can understand from its manual, it only has a global Jumbo enable/disable setting and no per-port or per-vlan MTU settings.

I have two VLANs on that switch:

  • a generic LAN for internal traffic, untagged on ports 1-4, which must be kept at MTU=1500
  • an iSCSI-dedicated LAN, untagged on ports 5-8, which could take advantage of Jumbo Frames

The four servers (two as a DIY iSCSI HA cluster made with pacemaker and IET that will provide storage for virtual machines, two that will run the virtual machines) each have three gigabit interfaces: one for the LAN (connected to ports 1-4), one for iSCSI (to ports 5-8) and a third one for the public network / WAN (connected to a different switch)

Is it possible to enable Jumbo Frames globally on the switch (ports 1-16), and on every SAN device (the four dedicated ethernet cards in the four servers), but keep LAN devices at standard MTU?

In theory the switch should not create / rearrange frames so the generic LAN will not get exposed to big/jumbo frames.

And yes, we plan to rework the infrastructure with something better. We'll probably throw away the DIY iSCSI cluster and put an MD3220i or similar thing, keeping the PC2716 as a generic LAN switch and connecting the iSCSI links directly from the servers to the SAN... but not today. Probably 2013Q4 or 2013Q4 if everything goes well, but for now I have to live with this hardware and get the most out of it.

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Well, first off, Jumbo Frames are very useful in iSCSI network but mostly if you are using software initiators or software targets (or both).

Your iSCSI network will still work even if the frames are smaller (1500 bytes). What you will notice is that with smaller frames, the CPU usage on both the initiator side and the target side will be higher than if Jumbo Frames where used.

So if you will change your switch for a dedicated one with Jumbo Frames enabled soon, you could simply wait.

On the other hand, I believe your assumption is correct that your regular LAN will not be affected.

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  • I never heard about this difference in usefulness of Jumbo Frames between hardware and software iSCSI devices. Do you have any pointer to articles or literature about that?
    – Luke404
    Jan 11, 2013 at 0:06
  • I had the same information in EquaLogic documents, but I can't link to it since it requires an account. But IBM does mention the same thing here.
    – Alex
    Jan 11, 2013 at 0:13
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I am interested if you are using this switch as both a front and a backend for a virtual cluster. You would be correct in believing that they are able to coexist like you are describing. The MTU is set on the clients you connect to the switch not by the switch itself. The switch just decides if it can do it or not. That being said the best way to check this would be to set up two machines on the switch in the non-iSCSI VLAN and send data back and forth while doing a packet capture. Then run a diff on the two packet captures. This will show you any differences.

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  • I added some details to the question clarifying the whole mess :)
    – Luke404
    Jan 11, 2013 at 0:04
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The key point in all of this is that all of the hosts - and the routers - on a given L2 subnet (i.e. a VLAN) are configured with the same MTU and that this MTU is greater than or equal to this value. A mismatch between hosts on the same VLAN can be a real mess.

Also - the primary mechanism that hosts with different MTU's have to allow for proper communication is PMTU discovery, which assumes both L3 separation and that ICMP unreachables are allowed.

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