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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Fundamentals and Operation

PPP Components and General Operation 1 2 3 PPP Standards

Differentiating LCP and NCP Links

Remember that the basic link is established by LCP, and NCP links are set up within the LCP link. Closing an NCP link does not cause the LCP link to be closed. Even if all NCPs are closed, the LCP link remains open. (Of course, no data can be passed until an appropriate NCP link is re-established; a device is required to discard frames received containing any layer three protocol that does not have an opened NCP.) To terminate a PPP connection, only the LCP link needs to be terminated in the Link Termination phase; the NCPs do not need to be explicitly closed.

A summary of PPP phases can be found in Table 24. In the table, the LCP Link Status and NCP Link Status columns show the status of the link as the phase starts.

Table 24: PPP Phases

Phase/ State	Phase Summary	LCP Link Status Upon Entry To Phase	NCP Link Status Upon Entry To Phase	Transition Requirement	Transition To Phase
Link Dead	Default state; physical layer not connected.	Closed	Closed	Successful physical layer connection	Link Establishment
Link Establishment	Physical layer connected, basic configuration of link performed by LCP.	Closed	Closed	Successful negotiation	Authentication
				Unsuccessful negotiation	Link Dead
Authentication	Basic link is now opened, and optional authentication of device is performed.	Open	Closed	Successful authentication or no authentication required	Network-Layer Protocol
				Unsuccessful authentication	Link Termination
Network-Layer Protocol	One or more NCPs open an NCP link within the LCP link.	Open	Closed	All NCPs opened	Link Open
Link Open	Link is open and operating normally.	Open	Open	Link failure or close request	Link Termination
Link Termination	LCP link is shut down.	Open	Open		Link Dead

PPP Components and General Operation 1 2 3 PPP Standards

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