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 Post subject: Help! My whole network seems capped at 1300 MTU
PostPosted: Wed Feb 16, 2011 9:10 am 
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Joined: Mon Apr 23, 2007 3:05 pm
Posts: 70
Location: New York, NY
I am at a loss here and help would be greatly appreciated.

To start, here is my network configuration:

Cable Modem <-> Linksys WRT54G <-> Netgear GS105 (gigabit switch) <-> wired devices (disregard wireless devices for now)

Two of the devices (a Win7 desktop and a NAS) are gigabit; the others (a printer and a laptop) are fast ethernet. I have the gigabit devices connected through the gigabit switch so that the desktop and the NAS can talk to each other using gigabit (which they do; transfers between them are faster than 100Mbps). The printer is connected directly to the router.

Here's my issue. The gigabit switch is supposed to support jumbo frames up to 9K, so I set both the desktop's and the NAS's MTU to 9K. To test it, from the desktop I pinged the NAS using "ping -f -l 9000 <ip>" and got the fragmentation message. I then tried pinging with 1500: same message. I have to go all the way down to 1272 (i.e., 1300 counting the TCP overhead) to ping without fragmentation! I also tried pinging the laptop from the desktop and got a max of 1272 as well. And vice versa. And pinging the NAS from the laptop. All combinations lead to 1272.

Thinking it might be a problem with the gigabit switch, I connected the desktop and laptop directly to the router--same result. The router's MTU setting is the default: 1500 (which shouldn't have mattered anyway when the devices were on the gigabit switch, bypassing the router).

Finally, I tried DSLreports' tweak tool (which looks at your WAN connection) and it indicated that I was maxed out at 1300. In fact it suggested that I "raise my MTU to 1500," which means that, from their end, it seems that I'm manually dialed back to 1300. But again, even if the cable modem or router was set to 1300, it wouldn't have affected LAN-only transfers through the gigabit switch. (Or would it? The router is the DHCP server--could it be throttling the computers when it assigns thier IP addresses?)

I started off wanting 9K jumbo frames but now I'll settle for 1.5K normal frames! Does anyone have any insights?


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 Post subject: Re: Help! My whole network seems capped at 1300 MTU
PostPosted: Fri Mar 04, 2011 11:35 am 
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Joined: Fri Mar 07, 2008 2:11 pm
Posts: 24
Location: Holland
Found this:

Quote:
Some models of the GS105 support jumbo frame and some don't, and you can determine which is which from the serial number - if your switch has a serial number starting with 1FE then it supports jumbo frame upto a maximum size of 9216 bytes.

Here: http://forum1.netgear.com/showthread.php?t=29068


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 Post subject: Re: Help! My whole network seems capped at 1300 MTU
PostPosted: Thu Mar 24, 2011 6:27 pm 
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Joined: Mon Apr 23, 2007 3:05 pm
Posts: 70
Location: New York, NY
That's a good indicator of why it won't do jumbo frames, but it doesn't explain why it would be capped at 1300.


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 Post subject: Re: Help! My whole network seems capped at 1300 MTU
PostPosted: Fri Mar 25, 2011 3:54 am 
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Joined: Thu Apr 21, 2005 12:17 am
Posts: 105
Location: Helsinki, Finland
Some months ago I upgraded my home server and desktop from onboard / USB-connected 1Gb ethernet to Intel PRO/1000 PT. At that time I also did some superficial research into jumbo frames, and simply decided it was absolutely not worth it.

The main issue is that in order to use jumbo frames you need to have support for that frame size in all the devices in the same subnet. So if you want to have your (presumably) non jumbo-supporting older computers in the network also, you need to have them in a different subnet and route between the subnets.

Also, the performance benefit between normal and jumbo frames is pretty much nonexistent for me since I have plenty of processing power on both communications endpoints. I get >80MB/sec transfers for big files, so I have no complaints. Even with a NAS the benefit of jumbo frames is only around 10% or so, if I remember correctly.


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 Post subject: Re: Help! My whole network seems capped at 1300 MTU
PostPosted: Mon Mar 28, 2011 7:29 am 
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Joined: Mon Apr 23, 2007 3:05 pm
Posts: 70
Location: New York, NY
mellon wrote:
The main issue is that in order to use jumbo frames you need to have support for that frame size in all the devices in the same subnet. So if you want to have your (presumably) non jumbo-supporting older computers in the network also, you need to have them in a different subnet and route between the subnets.


Okay, so if what you're saying is correct, then it's a "weak link" problem where one non-jumbo device on the subnet will spoil it for the rest. Maybe the same thing is causing my 1300 problem--i.e., some device is capped at 1300, and propagating this limitation to all of the other devices on the subnet? Could it even be the cable modem upstream of the router doing this?


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 Post subject: Re: Help! My whole network seems capped at 1300 MTU
PostPosted: Wed Apr 04, 2012 6:44 am 
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Joined: Fri Feb 10, 2006 2:50 pm
Posts: 675
Location: Milwaukee, WI
jbw wrote:
one non-jumbo device on the subnet will spoil it for the rest.

Exactly. Could be any non-gigabit device in your LAN. You can also avoid the mixed-network problems by segregating the jumbo devices on their own vlan. But your Netgear GS105 is unmanaged.

Like mellon, I quickly concluded that jumbos were not only not worth the effort, but pointless for my home LAN. I think jumbos had more relevance years ago - computing power was lower, as it was desirable to minimize how frequently the tcp/ip stack had to be climbed.

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