Specify the interval between L2TP client and server.
To check the connectivity of a tunnel, the client and server regularly send
PPP Echo to each other. If the client or server receives no response from
the peer within a specified period of time, it retransmits the PPP echo. If
it receives no response from the peer after transmitting the PPP echo for
max retries times, it considers that the L2TP tunnel is down and tries
tore-establish a tunnel with the peer.
You can enter some other PPP initialization strings in this field. Each
string can be separated by a space.