Cisco 3750X ISCSI Optimization / Jumbo Frames


I have a stack of 3x Cisco 3750X switches configured in a Stack. I have 3x VMware hosts with only a few VMs runnings. And a Dell PowerVault MD3200i which we are implementing.

I want to optimally configure the 3750X for ISCSI with Jumbo Frames and Flow Control and whatever else is needed. I found this guide for Dell Equallogic Optimization which I'm guessing would work great for the Dell Powervault ISCSI.

However, I have a few questions on this as I'm a bit rusty on ISCSI and Jumbo frames and haven't done it in a number of years:

1) The 3750X switch seems to only support enabling Jumbo frames switch wide. VMware seems to only support Jumbo up to 9000 Bytes. So that's what I'd use on the 3750X. However, I can't remember what the repercussions are on all of the other devices connected to the switches from routers, other servers, etc. Since they'll still be operating at MTU 1500, that will work just fine right?

I seem to remember that it only didn't work the other way round, if your MTU is set to 9000 but one of the points on the way is 1500, then it wouldn't traverse.

I just want to make sure that by enabling Jumbo frames on these switches (these are the only switches for all the devices) it doesn't take down the network.

2) In the link its got a bunch of QOS commands, I've never worked with QOS so a) don't know what these commands will do switch wide b) will they have negative effect on any other normal devices connected to the switches? Thanks!
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1) in addition you need to enable jumbo frames on all vmware virtual switches, then you can try enabling on hosts
2) Skip them for now, biggest problem is chopped ethernet frame tails

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You can set the jumbo frames to 9000 (or higher) on a 3750 connected to Vmware servers.  The jumbo frames setting on one switch will not take down the network, but you do want to check the other network devices that touch the 3750 and verify that they have jumbo frames enabled as well.  Otherwise traffic will remain limited to 1500.
   You can also enable flow control, although you probably won't notice any difference in performance unless your ports are constantly congested.
   And for QOS, you'd be better off setting QOS on the core switch or router on your network.  A SAN switch sits between the network core and server/storage, and cannot control the speed of incoming data via QOS.
RFVDBAuthor Commented:
Thanks very much for the answers.

I did some research and maybe you can just confirm the answers I've acquired:

I could change all my switches to MTU 9000 (Jumbo Frames) and still leave all of my endpoints, firewall, servers, VMware or not plugged into it and at MTU 1500 and it will continue to work. The switch just buffers everything at each port at 9000 instead of 1500 so anything under 9000 will work just fine. The switch won't reject anything and everything will continue to operate as normal - right?

However, if my switch was at MTU 1500 and the host was sending Jumbo Frames over 1500 like 9000, the switch would reject it, so the reverse is true, right?

Got it on the QOS, I'll just ignore that for now.
You start with switches, then go to vmware soft switches. at this point you can make 9000 sized pings, if they work - try first pair of end devices. Sending 9000 frames will chop the tail of frame on incompatible hardware, and small packets will work, like SSH, but big transfers will get stuck and time out as no useful packets arrive.
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