After upgrading my system to Ubuntu 11.04 I was no longer able to establish a VPN connection. Shrew Soft VPN would time out. It appears that the Reverse Path Filter was causing the issue.
Found the following solution:
Edit your network-security.conf to disable the Reverse Path Filter:
sudo vim /etc/sysctl.d/10-network-security.conf
Set the following items from 1 to 0:
Save and perform the following command.
sudo /sbin/sysctl -p