Max MTU Value differs for one system

[Chips Fowler]

I am trying to fix a laptop (W2K SP4) performance over VPN by
adjusting the MTU value. But I have found something very odd about
this system. When determining the MAX MTU for this system, I find
that the packets start to fragment at around 1345. But for any other
system on the local net, the Max MTU is found to be 1472 (as I would
expect).

To find the Max MTU value I used:

ping -n 1 -f -l <PacketSize> MyServer

and adjusted the PacketSize until ping failed because of the
fragmentation flag being set.

To try to isolate the problem, I hooked both the laptop and my system
to the same hub to try to find the Max MTU and my system was 1472 and
the laptop was 1345.

If packet fragmentation is caused by routers/devices on the network
and what can cause one system to have such a different Max MTU setting
than other systems on the same network?

Thanks,

-chips

 

[John Wunderlich]

To try to isolate the problem, I hooked both the laptop and my
system to the same hub to try to find the Max MTU and my system
was 1472 and the laptop was 1345.

If packet fragmentation is caused by routers/devices on the
network and what can cause one system to have such a different Max
MTU setting than other systems on the same network?

Is it possible that the MaxMTU setting on the laptop is currently set
near 1345 and this is causing the laptop to fragment differently than
your other machines?

-- John