L2TP dialup is successful after several attempts, but the system displays error 691.

After the AR is configured with L2TP, users need to dial up multiple times. During dialup, the system displays error 691. This is because the AR supports challenge messages with only 16 bytes. When the length of challenge message is not 16, CHAP authentication fails. The system displays error 691 (incorrect user name or password). To solve the problem, configure L2TP re-negotiation so that the LNS and client negotiate 16-byte challenge messages.

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