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We have a HP C7000 blade chassis with a HP Lefthand SAN solution. The ISCSI network is in its own VLAN and is using the standard MTU of 1500.

We also have a backup exec 2012 server that has read only access to the Datastores, and we use SAN based transport to backup the VM's directly off the lefthand. Everything works well.

Testing in my lab, I have found a large increase in backup performance when using jumbo frames. This is good news, I would like to configure the lefthands for 9000 MTU and configure the backup exec server for 9000 MTU also, but leave the ESXi hosts on the standard 1500 MTU.

All switches and network components in the paths have had jumbo frames enabled, just never used.

Thinking about it, I don't think this would be a problem mixing MTU's like this on a dedicated ISCSI network because the MSS is negotiated during the TCP 3 way handshake, and ISCSI works over TCP. There would be an issue if there was any UDP over that network, but I cannot think of any.

Can anyone offer any reason why I could not mix MTU's on a dedicated ISCSI network or any side effects that may result from it?

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  • Why would you leave your ESXi boxes with non-jumbo frames? Seems like a strange thing to do; if you can jumbo them you should.
    – Mark Henderson
    May 20, 2013 at 7:49
  • @MarkHenderson Correct, I would like to go jumbo frames for ESXi, but at this stage I haven't tested it yet in the lab, but it is on the cards, the question above is an interim step before going fully jumbo frames, provided the performance isn't impacted. Backups are 100% read, where the ESXi is 50/50 read/write.
    – user2060594
    May 20, 2013 at 7:58

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It's not really clear from your question whether you are asking whether it's OK to have an ESXi host that has multiple NICs set to different MTUs or not.

Your ESXi is connected to your iSCSI VLAN using a physical NIC. If the MTU of that physical NIC is 1500 yet the rest of your network is generating and sending jumbo frames, then those frames need to be fragmented in order to fit through your ESXi NIC.

I am not a networking expert, but I am reasonably sure that this fragmentation is done by your switch, so you are creating significant overhead on your switching equipment.

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