LCP Negotiation Parameters

LCP can negotiate many PPP options, as follows:

• MRU size—Maximum receive unit size (always accepted).
• Magic number—Randomly generated number used to identify one end of a point-to-point connection. Each side negotiates its magic number, taking note of each other’s magic number. If both sides discover that the magic numbers they are negotiating are the same, each side attempts to change its magic number. If they are not successful, and the magic numbers remain the same, the session terminates because of the loopback that is detected. Magic numbers are always accepted.

  By default, the router always attempts to negotiate a local magic number. The peer can also determine whether to negotiate its magic number—the peer magic number. The router always accepts a peer’s attempt to negotiate its magic number.

  If the peer does not attempt to negotiate its magic number, you can configure the router to ignore a mismatch of the peer magic number and retain the PPP connection. For details, see Validation of LCP Peer Magic Number.

• Authentication—Requested if configured.
• Protocol-Field-Compression (PFC) and Address-and-Control-Field-Compression (ACFC)—Accepted, but never requested.
• Multilink PPP—Additional options can be negotiated when Multilink PPP is configured. See Configuring Multilink PPP.
• Async-Control-Character-Map (ACCM—Supported by PPP when used with an L2TP Network Server (LNS). ACCM allows PPP to indirectly support asynchronous PPP connections tunneled via a third-party L2TP Access Concentrator (LAC). PPP on the router uses the ACCM configuration data as supplied by the LAC via proxy LCP. The router does not directly support asynchronous PPP connections and will not negotiate an ACCM option unless directed to do so by a third-party LAC.

PPP can also detect a loopback that occurs after LCP is negotiated, provided that:

• No loopback occurs during LCP negotiations.
• A loopback is introduced after LCP negotiation without forcing LCP renegotiation. (LCP is renegotiated if the lower layer goes down or if an LCP confReq is received from the other end.)

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