Using factory defaults, the Asus WL-600g router incorrectly handles packets responsible for VPN connection negotiation which are sent by Cisco VPN Client (perhaps this also holds true for other VPN software). Fortunately, the problem can be easily alleviated.
Just point your browser to the router’s administration page, on the left side choose: Advanced Setup → NAT → Port Triggering and then click the Add button on the panel on the right. Enter the following values in the form:
- Custom Application – select it and enter any name, e.g. “VPN”,
- Trigger Port Start – 500
- Trigger Port End – 500
- Trigger Port Protocol – UDP
- Open Port Start – 500
- Open Port End – 500
- Open Protocol – UDP
Click Save/Apply and everything should be fine from now.