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    Remote Peer Scenarios During Negotiation of PPP Options

    During a PPP configuration request, if any of the primary or secondary DNS option is rejected, or if they are unacceptable, the customer premises equipment (CPE) is prompted to negotiate the Internet Protocol Control Protocol (IPCP) primary and secondary DNS options that are locally available with the Broadband Remote Access Server (B-RAS). This provision is controlled by CLI and SNMP configuration options.

    The following describes the peer negotiations in different scenarios:

    • The CPE does not send the prompted options in the subsequent configure request
      • If B-RAS sends another NAK, it prompts the options again, until max configure-nak is exceeded
      • If B-RAS sends an ACK, it ignores the options and brings up the link
    • The CPE negotiates a different option from the prompted options
      • If B-RAS sends another NAK, it prompts the options again, until max configure-nak is exceeded
      • If B-RAS sends an ACK, it ignores the options and brings up the link
    • The CPE negotiates the prompted options but the option values are not acceptable
      • B-RAS sends another NAK with the prompted options, until max configure-nak is exceeded
    • The CPE negotiates the prompted options but some option values are not acceptable
      • B-RAS sends a NAK for unacceptable options, until max configure-nak is exceeded
    • The CPE stops responding on receiving the prompted options
      • B-RAS negotiation timer expires and the link is terminated
    • The CPE negotiates the prompted option after the link comes up
      • This is treated as a renegotiation request and B-RAS sends an ACK/NAK until max renegotiation and max configure-nak counters are exceeded, respectively
    • The CPE starts renegotiation without the prompted options
      • B-RAS renegotiates, until max renegotiation is exceeded
      • If B-RAS sends a NAK, it prompts the options, until max configure-nak is exceeded
      • If B-RAS sends an ACK, it ignores the options and brings up this link
    • The CPE NAKs or rejects the prompted options
      • This behavior is non-compliant with RFC because a configure-rej or a configure-nak must be sent only in response to a configure-req

    The Link Control Protocol (LCP) state machine stores the various phases through which an IPCP negotiation and link establishment occurs. When an IPCP configuration request is received from a CPE in the closed state (such as an IPCP request with an IP address of 0.0.0.0 received from the CPE) at the provider edge (PE) device, the LCP negotiation moves to the stopped state and the PE device does not send a NAK packet to the CPE.

    After an IP interface is created, which occurs when the PPP link is in the opened state and the PE device receives an open event, a Receive-Configure-Request (Bad) or RCR– event is sent to the CPE. This event occurs only if the previous event from the CPE was a close event or a bad IPCP configuration request was received. The RCR– event causes the configuration request and configuration NAK packets to be sent from the PE to the CPE device.

    Published: 2014-08-14