Causes for the L2TP dialup failure

The possible causes are as follows:
- L2TP packets are discarded because the firewall or the PC with built-in firewall on the public network discards L2TP packets.
- The corresponding ports of L2TP are disabled or occupied. Generally, UDP port 1701 is used. For example, ACL and NAT use this port.
- The user name and password are incorrectly configured on the LAC, or no user is configured on the LNS.
- The address is incorrectly configured. For example, the static address of the VT interface is incorrect.
- Tunnel authentication modes are different.
- LCP renegotiation is not configured.
- The addresses are improperly allocated. The address pool is small or is not set.
- The IP address pool is not configured with the gateway address, so that the gateway address is also allocated to the clients.
- Unreachable routes exist at both ends.
- In the L2TP group view, the specified remote tunnel name is incorrect.
- The authentication domain is configured incorrectly.
- Control packets sent by the built-in client of the PC do not carry the SQ serial number. As a result, L2TP negotiation fails.
- IPSec parameters configured at both ends are inconsistent.

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