[Openvpn-users] --mtu-test question

[Nik <op...@ba...>]

Hi,

I found that I was encountering what were probably MTU problems with my 
openvpn connection between my notebook and the conmpany server: 
downloads were fine, but uploads stalled after ~12kbytes.

Since my notebook connects using pppoe, I presumed the problem would be 
MTU on the notebook.

Rather than guess, I thought I should run openvpn with the --mtu-test 
option. This gave the result below:

Tue May 17 13:29:21 2005 us=609138 NOTE: Beginning empirical MTU test -- 
results should be available in 3 to 4 minutes.
Tue May 17 13:32:31 2005 us=205473 NOTE: Empirical MTU test completed 
[Tried,Actual] local->remote=[1541,1541] remote->local=[1541,1541]

These values surprised me, as I expected that PPPoE would cause a 
reduction in the local MTU to around 14xx??

I tried fragment and mssfix, but these caused the connection to be 
unusable. It would connect, but not even pings would get through.

Looking at ifconfig, I saw that ppp0 had an MTU of 1464, but that tun0 
had an mtu of 1500.

Adding link-mtu 1464 to the client config fixed the problems. But my 
question is:

Q. in this situation, are the results from mtu-test misleading, wrong, 
or being incorrectly applied/interpreted by myself?

Thanks in advance for any responses.

Cheers!
Nik.