Topology: FTP server――(internal network interface)AR28-31(external network interface)――public network――L2TP clients
L2TP clients dial in successfully, and they can download from FTP server, but they cannot upload to it.
Change the TCP MSS of relevant interface: interface Virtual-Template1 tcp mss 1400 interface Ethernet2/1 tcp mss 1400. The problem is solved.
In the checkup for configurations, the TCP MSS of internal network interface is set to 1400, and Virtual-Template1 tcp mss 1500. So it is concluded that the problem is resulted from that TCP MSS is different during the exchanging of FTP stream.