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MTU Settings

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MTU Settings

Hello everyone. I got an email from Plusnet advising me to reset my MTU etc. to the default Windows settings. I've been using an MTU of 1458, and a Receive window of 32614. I've been getting very good speeds with these settings, but I changed them back to the Windows default. My speeds decreased slightly by about 3KB up and down, so I changed them back again. My speed is now 60.1 down, and 30.1 up. Do these default settings apply to everywhere ? I should add that I live about 400 yards from the exchange, but I don't know if this has anything to do with it. I would be interested to hear of any similar experiences.
Regards, John Taylor.
7 REPLIES
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MTU Settings

Errr my adsl router (dg814) has a default setting of 1458mtu - however I seem to remember that when i first setup plusnet it was something different - I believe 1500

Can someone from tech support please confirm plusnets recommended and or default mtu
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MTU Settings

The default windows MTU is 1500.

The original problem stems from this setting being slightly too high for some of the BT machines to handle. As such, they have the ability to split the data into fragments.

Normaly this isn't a issue. However, these machines handle all the ADSL connections in the country, and splitting this data is becomes a very intensive process.

Because everyone normaly runs on default settings, it very easily bogs the BT machines down.

There are two fixes for this.

1: The MTU tweak, which you are now recomended to use

2: A router with a MTU setting.

Some modern firmware for router allow this, and some even come with it reduced by default to save tweaking it.

All this does, is the same as the BT systems. Split your data into fragments. However, because it is only doing it for your network and not the whole country, they don't get bogged down.

BT are currently running a exchange service program, to change the settings, hardware and software involved in the originating issue. They will increase the settings such that the default MTU (1500) does not cause these issues.

1500 is slightly more efficient than 1458, but not enough to see an imporvment.

If you are seeing a decline in service through this change, then there is one of two possible causes.

1: The receive window (RWIN) need tweaking again.

2: Your exchange has not yet been serviced to allow 1500 to operate fully, however, you should still see improvment over having it set at 1500 before any changes where made by BT.

I can only sugest waiting a few weeks, and try again. 1458 will not kill your system, and it wouldn't essentialy matter if left on 1458.

+Net have not been the best to keep people informed on the exchanges currently under the service program, however, there are other sources for that data.

While I am doing my best to get away from them, BT Openworld service status seems to keep me pretty informed on these service issues. You can see there status page HERE
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MTU Settings

Plus if you have a Sar 110 router it's MTU is fixed at 1452, so far no one has come up with a fix for this.
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MTU Settings

Quote
Plus if you have a Sar 110 router it's MTU is fixed at 1452, so far no one has come up with a fix for this.


This is true with the latest firmware for the SAR110, but is not a problem with the older firmware.

Tim
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MTU Settings

I can envision a new firmware release once BT have confirmed the issue as closed.
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MTU Settings

Thanks for the input evevryone. I did try tweaking the R window with an MTU setting of 1500, but I couldn't get any results as good as my original settings. I don't think there is much room for improvement to the speeds I am getting , so I think I'll leave well enough alone until something goes pear shaped. Once again, thanks for the input.
Regards, John Taylor.
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MTU Settings

Sorry. I forgot to mention that I don't use a router. I have a Samsung USB ADSL AHT-E310 modem.
Regards, John Taylor.