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More MTU questions

Grimbo
Grafter
Posts: 397
Registered: 23-05-2007

More MTU questions

Apologies because this is kind of discussed in another thread, but...

My router (Linksys wireless) has its own MTU setting which I guess is the MTU between it and Plusnet...

Then of course I have the Windows MTU setting which I assume is between my network card and the router (optimised at 1454)

The router is set to Auto MTU (defaults to 1500)... should I trust this of manually set it?

Many thanks.

Graeme
6 REPLIES
Community Veteran
Posts: 14,469
Registered: 30-07-2007

More MTU questions

2 things:

Your routers MTU should be set to the same as your PC (and all PCs should be set the same). It would not let it use the automatic mode to set it.

Where did you get an MTU of 1454 from? The most common options are 1478, 1458 or 1430 with the corresponding RWIN values (for Windows) of MTU-40 * 12 (where the 12 can be between 10 and 40).

This could have been asked in the MTU tweak thread for others to read as well.
Grimbo
Grafter
Posts: 397
Registered: 23-05-2007

More MTU questions

Thanks for the quick reply Peter....

Quote
Your routers MTU should be set to the same as your PC (and all PCs should be set the same). It would not let it use the automatic mode to set it.


Ok thanks.

Quote
Where did you get an MTU of 1454 from? The most common options are 1478, 1458 or 1430 with the corresponding RWIN values (for Windows) of MTU-40 * 12 (where the 12 can be between 10 and 40).


I used that ping -f -l xxxx www.plus.net where xxxx is the largest number that doesn't bring back an error then you add 28

The highest number I could use was 1426 (hence 1454)
Community Veteran
Posts: 14,469
Registered: 30-07-2007

More MTU questions

Did you do the test when both router and PC were set to an MTU of 1500 and both had been rebooted after the change? If not, that would be a false reading.

While that test may show the max MTU before packets are fragmented, it does not necessarily mean it's the best MTU for the fastest ADSL speed. This can only be achieved by trial and error. For me 1458 and RWIN of 17256 gives me the best speed, yours may be different.
Grimbo
Grafter
Posts: 397
Registered: 23-05-2007

More MTU questions

Quote
Did you do the test when both router and PC were set to an MTU of 1500 and both had been rebooted after the change? If not, that would be a false reading.

While that test may show the max MTU before packets are fragmented, it does not necessarily mean it's the best MTU for the fastest ADSL speed. This can only be achieved by trial and error. For me 1458 and RWIN of 17256 gives me the best speed, yours may be different.


Ok, i'm gonna try some of the examples to see which is best...

1478/63272
1458/62392
1458/17256
1430/61160

What would you suggest is the best way to test my settings after I have changed and rebooted PC and router (for both throughput speed and packet loss)

Cheers.
Community Veteran
Posts: 14,469
Registered: 30-07-2007

More MTU questions

Plusnets speed test (connection settings) and the speedtest on ADSLguide.

Also run the tweaktest on dslreports to make sure the MTU set is what is actually used (ignore the RWIN recomendation).

For packet loss use WinMTR to a number of local and remote servers.

And keep a note of MTU & RWIN values and the speed they achieved.
Grimbo
Grafter
Posts: 397
Registered: 23-05-2007

More MTU questions

Thanks mate, appreciate your help.

Graeme