I have an "interesting" transient problem with my laptops VPN connection - sometimes the VPN connection works perfectly, and other times it just doesn't work at all.
The Cisco VPN client software is version 4.8.01.0300. Operating system is Windows XP SP2.
Transport is configured with "Enable Transparent Tunnelling" and "IPSec over UDP (NAT/PAT)" selected.
When the connection fails, the client statistics window shows "Bytes Received: 0" and "Packets Decrypted: 0".
The weird thing is that the VPN can work properly for months and months, and then stop. Rebooting the system, has no effect.
This problem occurs for some other users as well, but never at the same time, and we've failed to solve the problem. Rather, it just seems to "go away", only to recur later.
At the moment, I have a partial failure. Using my mobile PC card, I can go online at 115kb/s and the VPN works well, though slowly. But, going online through my 802.11g wireless fails.
Given that things can work properly for months at a time, and that other users have flawless access while I experience this problem, I'm pretty sure the issue is local to my machine.
Through extensive googling, I found a suggestion to tune my MTU using ping to probe the connection. However, I found that both my mobile card and wireless connections maxed out at the same figure - 1272 bytes - so I'm guessing that MTU isn't the source of the problem.