H3C Technologies H3C SecPath F5020 - Manual (Page 55)

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Analysis and solution
Possible reasons for the data transmission failure are as follows:
• No route is available. The LAC must have a route to the private network behind the LNS, and vice
versa. Otherwise, data transmission fails. You can use the display ip routing-table command on the
LAC and LNS to check whether the expected routes are present. If not, configure a static route, or
configure a dynamic routing protocol.
• 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 unstable, the LAC and LNS might be unable to ping each other.