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I currently have the following network configuration

  • Apple Time Capsule (Gigabit Ethernet router)
  • 2 Windows desktop wired with Gigabit ethernet
  • 1 MacBook Pro in wireless N mode
  • 1 iPhone (obviously wireless)

I bought a NAS and a Gigabit switch to extend my network and the nas asks me if I want to enable jumbo frames for the network connection. So my question, could I?

I think if I do that I will have problems for wireless clients and I need to reconfigure both the desktops and the time capsule to use jumbo frames.

Should I add a router instead of a switch to segment the network in two parts in order to use jumbo frames?

Does this have any sense? Will performance improve with jumbo frames?

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2 Answers

up vote 3 down vote accepted

Jumbo frames means a larger than normal MTU. Usually the MTU is set as 1500 (or 1492), at least for 802.11 networks.

Each network device will have an MTU size set on it. Any frame that arrives at a device larger than its MTU will get fragmented.

So you would need to make sure the MTU on all your network devices was set at or more than the MTU on the NAS resulting from setting Jumbo Frames (probably 9000, but check your documentation).

Given the size of your network, I doubt whether you will get any noticeable performance increase from setting higher MTUs everywhere. At best, backups might run a little faster.

Update: I answered a question on testing whether jumbo frames are working. The answer might be useful in your research on this.

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Does the fragmentation happens transparently? Where is it handled? In the switch or directly on the client? –  Fabio Jul 28 '11 at 13:52
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@Fabio it is transparent to your desktop and all other devices. Fragmentation would happen on the first device that the jumbo frame hit and that does not support bigger MTUs. I agree with dunxd, I doubt you will notice any performance increase with jumbo frames. –  Alex Jul 28 '11 at 14:16
    
@Alex I'll give it a try and benchmark performances. I'd expect some increase when moving large files. We'll see. –  Fabio Jul 28 '11 at 14:57
    
Read up on fragmentation here en.wikipedia.org/wiki/IP_fragmentation. I'm sure you will be able to measure some level of performance improvement, but whether you will notice is what I would question. You are only pushing data around a small network with a small number of users. It probably isn't worth the effort apart from the educational value of course :-) –  dunxd Jul 29 '11 at 15:11
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You can certainly enable jumbo frame support, that way if you go out of your way to enable JF on your wired machines you may see a performance boost - it being enabled won't slow down standard frame traffic. Of course it won't work at all over the wifi interface as it's not supported but I see no harm switching it on, even if you don't use it.

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So if I turn on them only on clients which support them there won't be any problem on wireless clients? If I transfer big files from my notebook (wireless) to the nas it will handle mtu discrepancy in a transparent way? –  Fabio Jul 28 '11 at 13:51
    
No, jumbo frames is an ethernet standard, not a wireless ethernet standard. The link from notebook to base-station is standard frames only, I don't know the airport thing you have but hopefully it'll do the frame conversion, if it does then great, if not then switch it off. There's only one way to know for sure, give it a try. –  Chopper3 Jul 28 '11 at 13:53
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