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

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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.

Other related questions:
Does L2TP support NAT traversal
L2TP supports NAT traversal. L2TP uses UDP port 1701, so the destination port must be port 1701 when NATis used.

How to locate the mapping failure problem after the NAT server based on the IP address and port number is configured on an AR
After the NAT server based on IP address + port number mapping, the mapping fails. Use the following method: 1. Check whether the internal network server can be accessed. You can access the internal server through the intranet. 2. Check whether there are reachable routes from the external host and internal server to the NAT server. 3. Check whether the NAT server is correctly configured. 4. Check whether the mapped external port number is available. Replace the external port to determine whether you can access the internal server. 5. Run the display nat session command on the NAT server to check whether there are entries before and after mapping, and obtain packets to check whether the NAT server translates the address of data packets of external user access.

How to configure port mapping on an AR that acts as a twice NAT server
For details about how to configure port mapping of twice NAT server on an AR, see "Internet Access > NAT > Example for Configuring NAT to Connect Intranet Users to the Internet, Provide the Web Server, and Enable Intranet Users to Access an Internal Web Server Using Domain Names" in the Typical Configuration Examples.

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