Cannot connect to VPN after sleep - still works on Windows machine

Question

Hi Guys,
I'm very new to using VPN which I now need for work. I recently set up a new connection and it was working fine for a few hours until I closed my MBP lid and sent the machine to sleep. Upon waking the Mac up again the VPN connection appeared to be lost. Trying to reconnect gives me the following error:
The L2TP-VPN server did not respond. Try reconnecting. If the problem continues, verify your settings and contact your Administrator.
I've spoken to the sys admin at work and he can't find any problems on his side and the same VPN details appear to work fine on multiple Windows machines. I've tried creating a new connection with the same details and deleting the old one with no success. Tried restarting the machine as well. What other things can I try to potentially get this issue solved?

Answer 1

I tried BDAqua's suggestion. It did not work.
System Preferences/Network will not allow a MTU smaller than 72, so I was unable to try a MTU of 53. Maybe there is a configuration file that can be edited manually, bypassing the GUI tool? I tried several other MTU sizes, including 72, no joy. I reset the MTU back to 1492 (to match the router setting) and eventually was able to connect. Running ifconfig in Terminal shows that en0, after connection, still has a MTU of 1492.
I'm not sure that Mac OS X's dislike of PPPoA has anything to do with it, as this machine never sees PPPoA, it only sees standard TCP/IP - Ethernet. PPPoA is only, best I know, used between the router and the ISP, and no computer on the network cares or is even aware that something such as PPPoA exists. During the time I am unable to connect to the outside with this machine, I am also unable to connect on the inside, despite DHCP on the router having assigned an IP address to the en0 interface.