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Old 27th June 2008
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ai-danno ai-danno is offline
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Quote:
Originally Posted by Seb74 View Post
Will soon get my new 8-port GB-switch, and from what I've heard without jumbo frames you dont notice THAT much of a difference.
actually you should see a noticeable difference even without adjusting the MTU from the standard 1500 (1518 considering CRC and header.)

Quote:
Originally Posted by Seb74 View Post
I guess its best if all devices on the same subnet use the exact same MTU-setting for the frames...right? Jumbo frames ain't something magical, its just the ability to set a higher MTU-value....right again?
You are correct that "its best." If the machines are connected via this new-fangled switch you mention, then the switch itself needs to be able to accept frame sizes as large as the machines that are trasmitting to it. If this is not the case, the result will be that the frames larger than the configured port(s) on the switch will be dropped by the switch. This can have precarious and unintended consequences. I don't know personally whether a switch would break down large frames it can handle from one host before transmitting them to another host that isn't configured for that frame size, but it's my opinion that it should (at least on the 'nicer' switches.) This also assumes the switch in question can adjust MTU size per port, as opposed to a global mtu setting, which many cannot do.

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So, had a look in Vista and I can set as highest 7KB MTU (6KB 5KB 4KB....).
I guess that doesn't mean 7000 byte, but rather 7168 bytes???
This article suggests that with an 'elevated command prompt' you can set the MTU for an interface on Vista in terms of bytes, not KB. Again, be mindful of header and CRC for padding.

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So, do I just run "ifconfig vr0 mtu 7168" on my OpenBSD and thats it?
How do I even know 7168 is a supported size on that NIC?
My Vista machine didn't go higher than that for example, maybe others just go to 6KB or 5KB or....???
You know what size MTU a particular NIC supports by researching it's capabilities via NIC vendor documentation. As stated previously, NIC MTU sizes vary greatly, and not just from vendor to vendor, but also intra-vendor (model to model from same vendor.)

If your OBSD box was the switch itself, then it's not bad to set the MTU higher than anything that directly connects to it (This would also assume the OBSD switch would primarily be passing traffic from host to host, not originating much on it's own, and what it does originate is not too large for the attached hosts.) However, since you are getting a shiny new switch (what vendor/model?) then you will be better served by matching MTU sizes with the other hosts that will connect to the switch.

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What will happen if the values dont match correctly, is it just bad performance or will I loose connection to my BSD-server until all partys have the exact same value entered?
You likely won't lose the connection itself, just those frames transmitted from one host to another that are too large for the receiving host. Simple pings likely wouldn't reveal this vulnerability until you adjusted packet size up.

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Might have the BSD-machine on its own subnet (either VLAN or a totally separate interface and then jumbo frames wont do any good I guess, through the router, but I'll see how I'll set it up).
Respectfully, I have no idea what you are talking about with that last comment. Basically, research the MTU capabilities of all the NICs of all the hosts connecting to the new switch, research the MTU capabilities of the switch itself, then set the MTU to a high level that isn't too high for any of the players and you should have much joy.
Quote:
Thanks
Don't thank me until you try what I say and it actually works lol
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