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    Understanding PPP Link Control Protocol

    PPP’s Link Control Protocol (LCP) establishes a PPP link by negotiating with the PPP peer at the other end of a proposed connection. When two routers initialize a PPP dialogue, each router sends control packets to the peer. The control packets contain a list of LCP options and corresponding values that the sending peer uses to define its end of the link, such as the maximum receive unit (MRU).

    LCP negotiations continue until the peers either converge (that is, reach an agreement about values for connection parameters) or abandon attempts to establish a connection.

    If you configure a PPP interface without an IP interface or profile, the router negotiates LCP, but then terminates LCP after 2 to 3 minutes. Previously, the behavior in such a circumstance was to negotiate LCP and then leave LCP open.

    For static PPP interfaces, whenever LCP achieves a stopped state because of termination, negotiation failure, or some other cause, it goes into passive mode and waits for the other side of the connection to restart the negotiation process. Once in passive mode, the router periodically attempts to negotiate with the other side according to an exponential timeout algorithm.

    For static PPP interfaces, the router waits 15 seconds, attempts negotiation, waits 30 seconds if it fails, attempts negotiation, waits 60 seconds if it fails, and so on. The timeout periods are 15 seconds, 30 seconds, 60 seconds, 2 minutes, 4 minutes, 8 minutes, and 15 minutes. Once it reaches the 15-minute timeout, the router attempts negotiation every 15 minutes until successful. When LCP reaches the open state, the timer resets to 15 seconds.

    Dynamic PPP interfaces are always torn down when LCP achieves a stopped state. For more information, see Dynamic Interfaces Overview.

    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 MLPPP Overview.
    • 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.)

    Validation of LCP Peer Magic Number

    If the peer has not negotiated an LCP magic number, you can configure the router to ignore a mismatch of the LCP peer magic number and retain the PPP connection.

    Previously, the router terminated a PPP connection with a non-conforming peer when it received LCP echo request packets or LCP echo reply packets from the peer with a magic number that did not match the LCP peer magic number on the router. This is still the current default behavior if you do not explicitly configure the router to ignore the LCP peer magic number mismatch if the peer has not negotiated the magic number and retain the PPP connection.

    Configuring the router to ignore the peer magic number mismatch and retain the PPP connection is useful if your network includes peers that send a non-null or invalid magic number in the LCP echo request and reply packets despite having not negotiated the magic number. In this situation, the router expects to receive a null magic number from the peer, and terminates the PPP connection unless you configure it to ignore the peer magic number mismatch and retain the connection.

    To configure the router to ignore the LCP peer magic number mismatch and retain the PPP connection, use the ppp magic-number ignore-mismatch command from Interface Configuration mode or Subinterface Configuration mode. For more information, see ppp magic-number ignore-mismatch.

    To verify configuration of LCP peer magic number validation on the router, you can use the show ppp interface command. For more information, see Monitoring PPP Interfaces.

    Keep the following points in mind when configuring the router to ignore the peer magic number mismatch and retain the PPP connection:

    • If the peer negotiates the magic number but sends the router an LCP echo request or reply packet that contains a null or invalid magic number, the router strictly terminates the PPP connection. The router can ignore a mismatch of the LCP peer magic number only when the peer has not negotiated the magic number.
    • Using the ppp magic-number disable command to disable negotiation of the magic number on the router does not affect validation of the peer magic number. When you issue the ppp magic-number disable command, the router sets only the local magic number to null, but does not change or validate the peer magic number. (For more information, see ppp magic-number disable.)

    You can also configure validation of the LCP peer magic number for static MLPPP interfaces, dynamic PPP interfaces, and dynamic MLPPP interfaces. For more information about configuring static MLPPP interfaces, see MLPPP Contextual Command Differences. For more information about using profiles to configure dynamic PPP and dynamic MLPPP interfaces, see Dynamic Interface Configuration Using a Profile.

    Published: 2014-08-14