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As recently discovered, my ISP is currently using PPPoA in order to authenticate my ADSL connection.

I would like to know the optimal MTU Size to use for this connection?

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    You shouldn't need to worry about that. You can set 1500 from your router's WAN to your modem's LAN, but your modem's WAN interface should already be set and you shouldn't change it. The MTU of the modem's WAN really would depend on the hardware you're syncing with on the other end, but most use the default. – MaQleod May 27 '14 at 20:22
  • @MaQleod Err, I had trouble downloading a file, I called the router manufacturer, we went to the router config uage, it was on 1500, he said change it to 1400, we did and it fixed it. – barlop May 27 '14 at 20:49
  • DSL reports: How do I find my optimum MTU setting? – Ƭᴇcʜιᴇ007 May 27 '14 at 20:54
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MTU and Windows and Defaults: Windows default MTU is 1500, or 576 for external networks. 1500 is OK unless you are running PPPoE, want to use IPSec (Secure VPN's) or both, then it's too big. 576 is too small, and inefficient for broadband Internet!

Procedure to find optimal MTU: If your MTU is currently set too low, for example 576, the following procedure will not be able to detect whether you have discovered the "Optimal MTU size". First reset the MTU setting of your equipment to 1500, the maximum size it could possibly be. For PPPoE, your Max MTU should be no more than 1492 to allow space for the 8 byte PPPoE "wrapper”. 1492 + 8 = 1500. From there it is possible to experiment and find the optimal MTU value. For PPPoE, the stakes are high: if you get your MTU wrong, you may not just be sub-optimal, things like uploading files, or the loading of web pages may stall, or not work at all! The ping test we will be doing does not include the IP/ICMP header of 28 bytes. 1500 – 28 = 1472. Include the 8 byte PPPoE wrapper if your ISP uses PPPoE and you get 1500 – 28 – 8 = 1464. The reason for these numbers will be apparent very soon. To find out if your packets are getting fragmented, we use a Ping command from the command prompt. The best value for MTU is that value just before your packets get fragmented. Add 28 to the largest packet size that does not result in fragmenting the packets (since the ping command specifies the ping packet size, not including the IP/ICMP header of 28 bytes), and this is your Max MTU setting.

Here is what to do:

Windows users:

Go to Start/ Programs/ Accessories/ Command Prompt and type (or Copy and Paste) the following:

If you connect to your ISP using PPPoE: ping -f -l 1464 www.dslreports.com If you are not connecting to your ISP through PPPoE: ping -f -l 1472 www.dslreports.com (That is a dash lower case "L," not a dash "1." Also note the spaces in between the sections.)

Linux users:

If you connect to your ISP using PPPoE: ping -s 1464 www.dslreports.com If you are not connecting to your ISP through PPPoE: ping -s 1472 www.dslreports.com

OS X users:

If you connect to your ISP using PPPoE: ping -D -s 1464 www.dslreports.com If you are not connecting to your ISP through PPPoE: ping -D -s 1472 www.dslreports.com

Linux and OS X commands ARE case sensitive.

Next: Press Enter. If you get the "packet needs to be fragmented" error message, then reduce the initial 1464 / 1472 by 10 until you no longer get the "packet needs to be fragmented" error message. Then increase the setting by 1 until you are 1 away from (less than) getting the "packet needs to be fragmented" error message again. If you can ping through with the number at 1464 (for PPPoE) / 1472 (for non-PPPoE), with no error message, you are done! Stop. Add 28 to the highest number pinged with no error (for both PPPoE and non-PPPoE), and that sum is your Max MTU setting. Use that (calculated sum) Max MTU setting in your router, or on your PC if your PC connects directly to your modem.


References:

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Well, I have a PPPoA ADSL connection.

I had trouble downloading a file, I called the router manufacturer, we went to the router config page, it was on 1500, he said change it to 1400, we did and it fixed it. Maybe if it was a bit bigger e.g. 1410 or 1420 or more, it'd be more efficient and no problem . It shouldn't be more than 1500 that's for sure.

I'm not expert, but things may be efficient if it's bigger but not too big.

There is a great webpage that talks about finding your optimal MTU (perhaps more relevant if you have issues) http://www.dslreports.com/faq/5793

  • I'd bet the biggest that'd work was 1468. ;) – Ƭᴇcʜιᴇ007 May 27 '14 at 20:54

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