PPP link establishment process
Figure 1 shows the PPP link establishment process.
Figure 1: PPP link establishment process
Initially, PPP is in Link Dead phase. After the physical layer goes up, PPP enters the Link Establishment phase (Establish).
In the Link Establishment phase, the LCP negotiation is performed. The LCP configuration options include Authentication-Protocol, Async-Control-Character-Map (ACCM), Maximum-Receive-Unit (MRU), Magic-Number, Protocol-Field-Compression (PFC), Address-and-Control-Field-Compression (ACFC), and MP.
If the negotiation fails, LCP reports a Fail event, and PPP returns to the Dead phase.
If the negotiation succeeds, LCP enters the Opened state and reports an Up event, indicating that the underlying layer link has been established. At this time, the PPP link is not established for the network layer, and network layer packets cannot be transmitted over the link.
If authentication is configured, the PPP link enters the Authentication phase, where PAP, CHAP, MS-CHAP, or MS-CHAP-V2 authentication is performed.
If the client fails to pass the authentication, LCP reports a Fail event and enters the Link Termination phase. In this phase, the link is torn down and LCP goes down.
If the client passes the authentication, LCP reports a Success event.
If a network layer protocol is configured, the PPP link enters the Network-Layer Protocol phase for NCP negotiation, such as IPCP negotiation and IPv6CP negotiation.
If the NCP negotiation succeeds, the link goes up and becomes ready to carry negotiated network-layer protocol packets.
If the NCP negotiation fails, NCP reports a Down event and enters the Link Termination phase.
If the interface is configured with an IP address, the IPCP negotiation is performed. IPCP configuration options include IP addresses and DNS server IP addresses. After the IPCP negotiation succeeds, the link can carry IP packets.
After the NCP negotiation is performed, the PPP link remains active until either of the following events occurs:
Explicit LCP or NCP frames close the link.
Some external events take place (for example, the intervention of a user).
For more information about PPP, see RFC 1661.