L2TP dialup fails because the UDP port mapping of the NAT server is incorrectly configured

The AR is configured with port mapping so that packets from the public network are mapped to other network devices.
The troubleshooting for the L2TP dialup failure is as follows:
1. Verify that the configuration is correct.
2. If the configuration is correct, run the debugging ppp all and debugging l2tp all commands to collect debugging information for fault location.
3. If debugging information cannot be collected, check whether packets reach the LNS, are rejected by the LNS, and are forwarded to other network devices.
4. When port mapping is configured on the NAT server, consider service features especially L2TP and Telnet. Prevent service exceptions or interruptions.

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