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Understanding MTU

Community Veteran
Posts: 38,460
Thanks: 1,030
Fixes: 62
Registered: 15-06-2007

Understanding MTU

I need a bit of help from the experts round here.
I am not having any problems with speeds or sites but following on from another post I checked my MTU setting on my router which is 1458 but Windows 7 RC has it set to 1500.
What, if anything, is the likely effect of this imbalance.
I will add my vista readings when I reboot the computer this evening - Vista is 1500 also
Just to note my current sync speed is 3776 kbps
12 REPLIES
scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

Re: Understanding MTU

your router will be fragmenting the packets as its not alowing 1500 to pass through from your pc.
on the way back in they should be fine as your router sent them out at 1458
me thinks anyway
awaits for some one to blow my theroy out to space  Smiley
Community Veteran
Posts: 38,460
Thanks: 1,030
Fixes: 62
Registered: 15-06-2007

Re: Understanding MTU

Speed test give 380 - 381 kbps upload which doesn't appear to be too bad
scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

Re: Understanding MTU

spot on what i use to get think it was 383 the norm for my old adsl
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: Understanding MTU

a query came up the other day on  Thompson, had a long look at mine and could not find any mention of MTU, any idea where it is hidden?, I set mine a while back when using a 2110
Community Veteran
Posts: 18,806
Thanks: 265
Registered: 12-08-2007

Re: Understanding MTU

I downloaded Dr.TCP from here and set mine to the recommended figure:
http://www.dslreports.com/drtcp
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: Understanding MTU

thats OK for the PC what about the router? Sad
scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

Re: Understanding MTU

pierre think you have telnet the thompson to set mtu
is it the plusnet one?
4th reply here
http://www.pcadvisor.co.uk/forums/index.cfm?action=showthread&threadid=321740&forumid=1
if your runing vista you may have to install telnet as its not on by default and start the cmd in admin mode
Community Veteran
Posts: 18,806
Thanks: 265
Registered: 12-08-2007

Re: Understanding MTU

We'll have to wait for one of the more technically able guys to come on here Pierre to answer your question.
Community Veteran
Posts: 1,576
Thanks: 3
Registered: 13-04-2007

Re: Understanding MTU

To find out your mtu the old MSDOS way see
http://help.expedient.com/broadband/mtu_ping_test.shtml
People mess with MTU without understanding it each pc has its own mtu and it can be different depending the host you test. All you can do is get the best average as it may change next time you logon if you get  onto another gateway. Your pc will be effected by the router and most modern routers auto detect the best settings. Some ISP have their own funny setting like AOL
Community Veteran
Posts: 26,718
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Registered: 10-04-2007

Re: Understanding MTU

I have never seen any proof that setting the router to the maximum (1500) irrespective of the setting in the PCs is in any way detrimental. That means that whatever the packet size decided on by either end of the communication the router will pass it through unchanged.
I have seen throughput nearly halved for someone using wireless when the router MTU was lower than the PC MTU.
jelv (a.k.a Spoon Whittler)
   Why I have left Plusnet (warning: long post!)   
Broadband: Andrews & Arnold Home::1 (FTTC 80/20)
Line rental: Pulse 8 Home Line Rental (£13/month)
Mobile: iD mobile (£4/month)
Community Veteran
Posts: 26,718
Thanks: 931
Fixes: 10
Registered: 10-04-2007

Re: Understanding MTU

Quote from: Oldjim
I am not having any problems with speeds or sites but following on from another post I checked my MTU setting on my router which is 1458 but Windows 7 RC has it set to 1500.
What, if anything, is the likely effect of this imbalance.

If MTU path discovery is on and working correctly Windows 7 should be dropping back to use 1458 automatically. I think I read something in the past which suggested a router needed to be pingable for MTU path discovery to work fully - not sure if this is true.
jelv (a.k.a Spoon Whittler)
   Why I have left Plusnet (warning: long post!)   
Broadband: Andrews & Arnold Home::1 (FTTC 80/20)
Line rental: Pulse 8 Home Line Rental (£13/month)
Mobile: iD mobile (£4/month)
Community Veteran
Posts: 18,806
Thanks: 265
Registered: 12-08-2007

Re: Understanding MTU

I know how to check and set my PC MTU but how do I do the router?  I use a D-Link 2640B.