L2TP dialup is successful after dozens of attempts and error 691 is displayed. Why

After L2TP is configured on the device, users can dial up successfully after several attempts and error 691 is displayed during the attempts. The cause is that the device supports only 16-byte challenge messages. When challenge messages are not of 16 bytes, CHAP authentication fails and error 691 indicating user name or password error is displayed. Configure L2TP re-negotiation so that the LNS and client can negotiate the 16-byte challenge messages.

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