I set up an L2TP connection from one ROS router to another ROS router. The server can use IPSEC. The client does not use IPSEC, the connection will succeed, and the encryption method of MPPE128 is adopted. When I switch the client to IPSEC mode, the connection will succeed, using CBC (AES) + HMAC (SHA1) encryption. I disabled IPSEC again, and the MMPE128 encryption was lost when the connection was successful, using an unencrypted connection. Sometimes, this way can't be successfully connected anyway, until I restart the ROS router as a client, the connection will succeed (the server ROS router does not need to restart).
Hope this is a bug and can be improved because the connection is unstable and you have to manually restart client ROS.
(by google translator).