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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 Configuring Dynamic Interfaces.

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