What are possible causes for L2TP dial-up failures of the AR router

Possible causes for L2TP dialup failures are as follows:
- The firewall is configured on the public network or the local PC has the firewall, so L2TP packets are discarded.
- When corresponding L2TP port is disabled or occupied, UDP port 1701 is often used. For example, ACL and NAT are configured. - The user name and password of the LAC are incorrect, or no users are specified for the LNS.
- The configured address is incorrect. For example, the statically configured address of the VT interface is incorrect.
- Tunnel authentication modes are different.
- LCP renegotiation is not configured.
- The IP address allocation is improper. The IP address pool has a small address range or not configured.
- Gateway addresses are not configured in the IP address pool, so gateway addresses are allocated to clients.
- There are unreachable routes.
- In the L2TP group view, the specified tunnel name at the remote end is incorrect.
- The configured authentication domain is incorrect.
- L2TP negotiation fails because control packets sent by clients of the local PC do not carry the SQ.
- When IPSec encryption is used, the IPSec parameters on the two ends of the tunnel are inconsistent.

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