Interface Rates on Linux Firewalls

I was working with the Cisco SRP541w and noticed something strange when remote users connected to the VPN. Once a VPN connection was established and remote users attempted to download something from the office, the firewall would reboot itself. After a little bit if research the problem seems to be directly related to the QOS bandwidth control under network setup.

It does not matter what this is set to, if it is enabled on any of the interface the problem will still occur when IPsec VPN users connect.

The Cisco SRP541w is running a GNU/Linux OS and I have seen issues similar to this on other variant Linux operating systems. The Zentyal will suffer the same consequence when rates under traffic shaping is enabled.

Enabling this on the WAN interface will cause the OpenVPN users to crash the outside interface after establishing a connection. I have not tested the stability of the connection over PPTP VPN but I suspect it will do the same thing since the issue is related to the uplink on the WAN connection and is present in IPsec and OpenVPN.