Mtu

[ichilton]

Hi,

I'm using an ADSL modem and cable router to connect to O2 Broadband.

Please could someone tell me what the correct MTU to use is? - i've seen mention of both 1492 and 1500 and am not sure which to use.

Thanks,

Ian

[Saturday]

1500

[ichilton]

Hi,

Great - thanks!

(that's what I had).

Ian

[kam.ho]

I've just run a series of tests using this site as a target, and found that I had to reduce the MTU to 1472 to avoid fragmentation.

[ichilton]

humm, so are you saying I should set it to 1472?

How do you know if you are getting fragmentation?

Thanks,

Ian

[kam.ho]

Actually, I have just learned that you can 28 to the value detemined by ping tests to get the value to enter into the router. Therefore, 1500 is the correct answer.

[Saturday]

Yes, to calculate MTU find the largest package size that transmits unfragmented then add 28 (for the IP/ICMP headers).

Like so:

• at the DOS prompt, type in ping www.o2user.co.uk -f -l 1472 and hit the enter key
  
• now press the F3 key and change the last figure to 1473

That shows the largest packet size is 1472, add 28 for the headers and that gives you an MTU of 1500.

Put that figure in your router. If you are using XP then also set that in each PC via a tool such as DRTCP. If Vista then it should be set automatically.

[ichilton]

Great - thanks for clearing things up.

Ian