Why use MTU 1500 when ISP cant handle it

Doc_Teal · Post by **Doc_Teal** » Thu Oct 10, 2002 11:27 am

Just wondering, like for me and Mnosteele52 service provider Cox Communication cant handle a larger than 1470 MTU before it has to fragment the packets up to send them, so what the use of using 1500. I know using 1500 the packets will still get there they just require more handling and processing time. I found this out by doing this from a command prompt

ping -f -l 68.10.xxx.1 <--- use your ISP IP, The Cox cable Tech showed me this when he was here yesterday, thoughts on this guys?

mnosteele52 · Post by **mnosteele52** » Thu Oct 10, 2002 5:37 pm

It should handle 1500 no problem, if it doesn't you might have a line problem, call Cox and see.

earthmofo · Post by **earthmofo** » Sat Oct 12, 2002 2:56 am

If you use a 1500 MTU value the maxium amount of data in the packet will be 1460 (MSS value) bytes because it contains a 40 byte header.

When using the command "ping -f [don't fragment] -l [size] 68.10.xxx.1", [size] is the maxium amount of data that can be sent without fragmentation.

In other words, [size] should be set to your MSS value and not your MTU value.

neo86 · Post by **neo86** » Sat Oct 12, 2002 5:28 pm

Don't forget that the ip header when using ping -f -l command is only 28 bytes.

That's why mine will go to 1472. 1472+28=1500.

When using that system add 28 to the number and then subtract 40 for the mss to be used in your config.

earthmofo · Post by **earthmofo** » Wed Oct 23, 2002 1:37 am

Originally posted by neo86
Don't forget that the ip header when using ping -f -l command is only 28 bytes.

I didn't know that! Thanks neo