Yes, move ports off VLAN 1 when using Jumbo frames.  Just about all switches have this issue.   Delayed ACK and LRO should definitely be turned off.   Delayed ACK / LRO aren't "broken" but in iSCSI environments they can add artificial latency to IO.    Jumbo Frames are negotiated on a per session basis.  So they can be mixed w/o an issue.  So if you ESX v4 servers aren't configured for Jumbo Frames it won't change anything.   I.e. you can disable JF on the vSwitch and even if the VMK ports are configured for JF, it won't be negotiated.   During the login sequence the target send a SYN packet in JF to the server.  If that's ACK'd the iSCSI connection will be made with JF.  If that times out, another SYN packet with STD frames is sent.  When that's ACK'd you get a standard frame iSCSI session.   So everything in the path from array to initiator must support JF.  Any break in the chain and you get standard frame connections.  How many ports are trunked between the switches?    How many arrays?  Regards, 
from [URL] make sure you Dell switches, (6224's?) have current firmware and flowcontrol is enabled, ports, spanning tree portfast is set, and you are NOT using the default VLAN. Put all the ports on another VLAN like 11 for example. Jumbo frames don't work well on default VLAN.--- We have 2 5224 and 2 5248 in use for iSCSI (dedicated). Should we also set this custom vlan stuff before enable jumbo frames, or is it only for 62XX? We have current firmware and flow control enabled.  from the above and from [URL] ---Disable DelayedACK and LRO--- Is this issue still present? Is it fixed in some firmware/esxi patch?  I'm going to enable jumbo frames on the SAN network. I would like to start with the first esxi 5 server and migrate the vi4 hosts after some testing, but I'm little afraid about the running systems.If I just enable it on the pswitches, should this create issues on my old vi4 hosts where I didn't enable jumbo frames?The EQLs already shows MTU 9000 on the interfaces.Is there some sort of negotiation between the endpoints?If not, can I still mix Guest VM iSCSI without jumbo frames with using jumbo frames on the Hostlevel
best practice iscsi advanced settings (switches, vsphere)