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Should Jumbo Frames be enabled on a LAN

I am installing a new server and workstations at a site.  When this is done, all of the computers and switches will be able to handle Jumbo Frames, but the printers, router, and other devices will not.  There are also some machines connected via a VPN.  Most files are small to medium in size, and file transfer bottlenecks will affect productivity.

I find a lot of differing opinions on whether Jumbo Frames should be enabled.  Everything from it will speed things up to it won't make a difference to it will cause applications to not work right, especially across the VPN.

Does anybody know of any authoritative criteria on which to base the decision on whether enabling Jumbo Frames will be beneficial?
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Harold Krongelb
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Harold Krongelb
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Paul SolovyovskyCommented:
We typically follow the keep it simple model.  On the storage side (Netapp, EMC, HP, etc..) we enable jumbo frames because of large throughput. On the workstation side on a gigabit network the traffic is switches (not shared as in the old days with hubs) which means that you're not going to get that much benefit and not adding something may be better than adding something and adding complexity, especially when weird issues happen and you're scratching your head.
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RGRodgersCommented:
I'd recommend you describe your network in some detail.

I also have several basic questions.  What do you mean by small to medium in size?  How will file transfer bottlenecks affect productivity?  How often are files transferred?  Do you expect VPN to support Jumbo Frames when your routers do not?

...RG
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RGRodgersCommented:
When you describe the network, please include network speeds throughout.  ...RG
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Harold KrongelbAuthor Commented:
In response to the request for additional info:

1.  It is a small office environement with two servers (2003 and a new 2008), and 8 workstations., and three gigabit switches.

2.  The file size is typically 23K to 4meg.

3.  I do not expect Jumbo Frames to work across the VPN.  I just don't want any conflicts or network issues.

4.  Files are transferred constantly.

5.  Almost everything is 64-bit.  We have a Server 2003 R2 hosting Exchange 2007.  A new Server 2008 will act as DC, filehost, etc.  A third Server 2008 will run our voicemail system and backups.

6.  Peripherals include a typical mix of printers, WiFi stations, and NAS devices (the NAS supports Jumbo Frames, the other devices do not).

5.  The issue is delays in transfer and the users response to these delays.  (I have added a high-end server to try and address some of the issues, which is not yet hosting the files.  This question is how to configure the server and workstations).  While most transfers occur quickly, some are delayed by a few to as much as 10 seconds.  Since this is a transient issue, I have not been able to track down the cause.  When there are these delays, the users may re-issue a command.  If they don't notice this and undo it right quickly, the project they are working on can become corrupted, and they may have to go back and redo one or more hours of work.

I am hoping my new server and 64-bit workstations (replacing old XP machines) will go a long way to solving the problems.  I have just installed the server and one workstation to solve as many configuration issues s possible before the users switch over.  My question about Jumbo Frames is what is the optimal configuration of those machines.

Please let me know if there is any more info I cna provide.
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RGRodgersCommented:
So you have gigabit to the servers and desktops supporting jumbo frames throughout.  What frame size are you considering?

I am concerned about the delays unless you know the root cause.  Since that doesn't seem known, I'd think you would want to ensure that has been totally resolved before you implement something like jumbo frames which could exacerbate the situation or, alternatively, mask it and make it more difficult to diagnose.  Your new configuration might just eliminate it but we won’t know that until it is fully implemented, tested and proven.

Here is a good discussion on the topic: http://www.smallnetbuilder.com/lanwan/lanwan-features/30201-need-to-know-jumbo-frames-in-small-networks. Their testing uses 1GB files to validate jumbo frame performance, significantly larger than your transfers.

Another concern would be the voicemail system.  Are you using VoIP?  If so, jumbo frames could impact it.  From the reference above, “And if you have a high amount of latency sensitive traffic, such as voice, jumbo frames can be counterproductive, as they can add delay in filling the packets.”

Would you be using jumbo frames on both ends of the VPN or just the host side?  My concern about having them on both ends would be that they would attempt to send the jumbo frames across the VPN and PMTUD would be needed to resolve that situation.  In that case, retransmits could eliminate any performance gains or, in the case of UDP, cause packets to be dropped and entire jumbo frames to be retransmitted.  See the reference provided above.

Interesting quote, also from the discussion provided above: “Today's CPUs and bus speeds are faster and can handle more instructions and frames per second than those used to generate the chart in Figure 2. So, ironically, today's higher CPU and bus speeds have made utilizing smaller frames over gigabit Ethernet less of a problem for PCs. NAS devices, on the other hand, have slower CPUs and bus speeds, making NAS data transfers a main beneficiary of using jumbo frames.”

The reference shows how to test for maximum frame sizes.  It also makes a recommendation to divide the network into jumbo frame/non-jumbo frame segments.  Finally, it discusses the impact on devices such as printers on a jumbo frame enabled network.

All things considered, I am not 100% that jumbo frames will improve your network performance.  They might but there are a host of considerations regarding this change.  You’ll have to test and measure to prove it.

Questions?

…RG
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Harold KrongelbAuthor Commented:
It appears that the gains are minimal and the risk of problems is high.  It is not worth the potential problems.  As to the cause of the slowdowns, no luck in tracking that down yet.  I will resume after I have replaced the server and workstations, if the problem persists.
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RGRodgersCommented:
I think that is prudent...RG
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