Why would i suddenly need to adjust my MTU size to get my cable intertnet working again?

[aphex]

Following this thread.

I had to adjust my MTU size down from 1500 to 1470 on my router to get some sites working again after they stopped a few days ago....

What would cause that?

I only thought you usually had to adjust that when you are on DSL.

 

[spidey07]

doing any VPN? Anything that is added to the frame (IPsec encryption, authentication, etc) could cause that symptom.

 

[JackMDS]

Connect the computer directly to the Modem and ping with few MTU values to see which results in the best ping.

In DOS box use the command: ping -l 1472 -f www.dslreports.com (after the command ping it is dash small L).

You start with 1472 since ping does not use the 28Byte header.

Do this few times reducing the MTU number by 10 (1472, 1462, and 1452).

If 1472 is not he best then your Internet connection is to be blamed.

:sun:

 

[aphex]

Thanks for the info, 1472 is indeed the best.... However, on a cable connection, shouldnt it be able to go up to 1500?

When i just tried 1482, i got:

C:\Documents and Settings\Josh>ping -l 1482 -f www.dslreports.com

Pinging dslreports.com [209.123.109.175] with 1482 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 209.123.109.175:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

 

[JackMDS]

That is OK ping MTU is 28 Byte smaller.

So a ping of 1482 would be as though the system uses 1482 + 28 = MTU 1510.

If 1472 is the best then it means that your ISP does provide the correct connetion 1472+28= MTU 1500.

:sun:

 

[spidey07]

jack is dead on (and kinda what I was getting at) you have a fragmentation problem.

my guess is your ISP is using some kind of a tunnel and the added overhead of the tunnel is causing MTUs greater than 1472 to be dropped.

Call you ISP and tell them they have a serious problem. Somebody has broken what is called "path MTU discovery" and that reaks havoc with internet communications.

Tough to go into the details, but one should be able to ping with upto a 65,000 bytes without problems - if there are problems, something is wrong.