PPP session establishment progresses through three phases. These phases are link establishment, authentication, and the network layer protocol phase. LCP frames are used to accomplish the work of each of the LCP phases. The following three classes of LCP frames are used in a PPP session:

• Link-establishment frames are used to establish and configure a link.
• Link-termination frames are used to terminate a link.
• Link-maintenance frames are used to manage and debug a link.

The three PPP session establishment phases are:

• Link-establishment phase – In this phase each PPP device sends LCP frames to configure and test the data link. LCP frames contain a configuration option field that allows devices to negotiate the use of options such as the maximum transmission unit (MTU), compression of certain PPP fields, and the link-authentication protocol. If a configuration option is not included in an LCP packet, the default value for that configuration option is assumed. Before any network layer packets can be exchanged, LCP must first open the connection and negotiate the configuration parameters. This phase is complete when a configuration acknowledgment frame has been sent and received.
• Authentication phase (optional) – After the link has been established and the authentication protocol decided on, the peer may be authenticated. Authentication, if used, takes place before the network layer protocol phase is entered. As part of this phase, LCP also allows for an optional link-quality determination test. The link is tested to determine whether the link quality is good enough to bring up network layer protocols.
• Network layer protocol phase – In this phase the PPP devices send NCP packets to choose and configure one or more network layer protocols, such as IP. Once each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. If LCP closes the link, it informs the network layer protocols so that they can take appropriate action. The show interfaces command reveals the LCP and NCP states under PPP configuration.

The PPP link remains configured for communications until LCP or NCP frames close the link or until an inactivity timer expires or a user intervenes.

Interactive Media Activity Drag and Drop: Establishing a PPP Session When the student has completed this activity, the student will know the three steps in establishing a PPP Session.