This article describe, how to setup a VPN connection between a DD-WRT router and a TP-Link TL-ER6120 acting as the PPTP server.
Configuration of TL-ER6120 is very trivial, their is so little options it's hard to do a mistake. First, go to VPN > L2TP/PPTP > IP Adresse Pool. Create a new address pool range. Notice, this address pool must not conflict with local subnet nor the remote subnet. i.e.: 192.168.15.100-192.168.15.110
Go to VPN > L2TP/PPTP > L2TP/PPTP Tunel and create a new tunnel as follow:
Go to Services > VPN, change the PPTP client settings as follow:
The magic setting is the MPPE encryption. noipdefault let the pptp server provide an IP to the client. I figure out the mppe required,stateless by trial and error. Don't try with stateful option. The PPTP client doesn't support it.
If you try with this settings, the PPTP client will establish a connection, but will disconnect within the next minute. After investigation, I notice the PPTP client is creating a bad route to the remote IP forwarding all the trafic to the ppp0 interface.
Removing this route fix it all. To automate this process, go to Administration > Commands. Enter the following text and save it as startup script.
(while [ ! -f /tmp/pptpd_client/ip-up ]; do sleep 10; done cp /tmp/pptpd_client/ip-up /tmp/ipup.tmp sed '/^ kelokepptpd)/a\ /sbin/route del -host $5 dev $1' < /tmp/ipup.tmp > /tmp/pptpd_client/ip-up ) &