Enable jumbo frames on a port 3com 5500G switch


I want to enable jumbo frames on several port on a 3com 5500G switch. To improve performance of iscsi traffic to the SAN.

Does anyone knows how.

Thanks in advance

P Mee

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I would think there would be a document or two at

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would describe how.

Still, depending on your NICs, I'd think they'd already have support for TSO - TCP Segmentation Offload - unless they are really cheap NICs. I like JumboFrames, but unless the NICs don't have support for LRO (Large Receive Offload) it isn't clear that switching to Jumbo Frames is going to have all that large an effect. TSO has been described as "poor man's Jumbo Frames" because it can allow the sending TCP to pass up to 64K at a time to the NIC which then resegments, but the receiving TCP still gets a stream of standard size segments. LRO is the inverse (reverse?) to allow a receiving TCP to be given larger segments even though the MTU is still 1500 bytes.

Also, in broad handwaving terms, unless *all* your traffic is TCP (or a protocol that exchanges packet size limits) if you enable JumboFrames on one port in the broadcast domain you must enable it on

*all* ports in the broadcast domain. Otherwise you run the real risk of one host with a JF MTU trying to send UDP datagrams to a host with a standard MTU and sorrow and woe will be the result.

rick jones

My recollection is that Jumbo Frames on top of TSO+LRO is reasonably well into the "diminishing returns area. One thing though is that while TSO and LRO depend on CKO (ChecKsum Offload) JumboFrames do not...

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Rick Jones

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