H3C SR6600 Configuration Manual page 102

Possible reasons for data transmission failure are as follows:
3) The user address is set incorrectly. Usually, the address of a user is allocated by the
LNS. However it can also be set by the user. If the address set by the user is not in the
same network segment as that allocated by the LNS, data transmission fails. It is
recommended that addresses be allocated by the LNS.
4) Congestion occurs on the Internet backbone and the packet loss ratio is high. L2TP
data transmission is based on UDP, which does not provide the packet error control
function. If the line is not stable, the LAC and LNS may not be able to ping each other
and L2TP applications may fail.
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