jeremymacmull

Gigabit Switches

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Some clarifications on Jumbo Frames. (I don't claim to be a networking expert, but this is based on conversations with associates of mine doing research on high-speed interconnect for super-computing clusters).

Going from normal frames (~1500 bytes) to jumbo frames (~9000 bytes), isn't designed to get you more throughput. The idea is to reduce the number of frames send for a give hunk of data, so that the hosts have less interrupts to handle. While less frames, do mean less header data sent, unless the bottleneck is the network, it doesn't give you anything.

Looking at benchmarks of jumbo frame vs. non-jumbo frame isn't very helpful (in theory). What you want to be looking at is CPU utilization. Jumbo frames can reduce interrupt load by a factor ~5, which can be very helpful, especially if you've got multiple NICs going on the same machine.

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I didn't want to start a new thread.

More related Threads:

Dell Switch thread ---> http://forums.storagereview.net/index.php?...=19882&hl=jumbo

Jumbo + cable modem ---> http://forums.storagereview.net/index.php?...=18038&hl=jumbo

I found this switch and was wondering if anyone had any experience with it.

http://www.milan.com/managed_switches/s8001tg.html

Looks like a good price/feature.

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My understanding from previous discussions on this topic is that it's all or nothing for jumbo.  The switch doesn't have the capability of breaking up the jumbo frames when forwarding them on to NICs that don't support it.  And the NIC uses a constant frame size when sending data - it can't intelligently alter the frame size dependant on the recipient.

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What I'm suggesting is that since TCP/IP requires a handshake prior to operation, one could arrange matters so that the packet size is the best available prior to data transfer from one NIC to another via the switch. Ergo, a network could exist with both jumbo and non-jumbo frames. This is theoretically possible; I just don't know if it's practical or warranted.

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I think the problem is that frame size is a functionality of a lower level of the network model than TCP/IP. So the entire local LAN must be operating in either jumbo or non-jumbo mode, because even if the sending NIC could query the receiving NIC to check for jumbo capability (which would happen at the driver level), there's no guarantee or method to check the entire network path to the receiver supports jumbo. At least that's my understanding of why you can't mix jumbo and non-jumbo on a normal, single LAN.

Maybe you could use VLAN technology to get around this problem, but there's no consumer level switches that support both jumbo and VLAN. Or you could use a switch or router that operates at a higher level.

i have just tryed a netgear switch and i get better throughput from an adhok connection with 9014 byte packets(nic to nic 39m/s) with swith i have to set packet size below 1515 and then the throughput is 26m/s and cup usage goes up about double. read this page . i have checked out these SMC switches and although about $2000 for a 4 port switch. the switch will fragment your jumbo packet for any segment (vlan segment) on the network that does not support jumbo. but there will be a little overhead loss on the 1514 mtu segment because the 9014 packet does not break up into an even number of 1514 packets. this loss is very small but can add more cpu overhead than u might think on the smaller packetted vlan.check out this Jumbo Packets and why you should care

url for the SMC switch SMC8504C

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That Columbia link, while interesting, is 6 years old.

You can get >900Mbit out of gigE with a 1500 byte MTU. Which isn't to say that jumbo packets are a bad idea (the CPU utilization, even with today's fast chips, is significant without them), but to base your decisions on that data would not be a good idea anymore.

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I saw the Milan switch as well. You'd still need layer 3 switching support before you could GigE jumbo frames on a home network, without using dual NICs though.

Edited by DigitalFreak

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