Point to Point Protocol (PPP) Tutorial

Point-to-Point Protocol (PPP) is an open-standard Data Link layer protocol used to carry Layer 3 packets across a point-to-point link. It works over both asynchronous serial links, such as a traditional dial-up POTS connection, and synchronous serial links, such as ISDN or a leased line.

PPP’s Two Sub-Protocols

• LCP (Link Control Protocol) — responsible for setting up and negotiating the options of the Data Link layer (Layer 2) itself.
• NCP (Network Control Protocol) — responsible for negotiating the parameters needed by whatever Network layer (Layer 3) protocol is actually going to run over the link, such as IP.

Phases of a PPP Session

1. Link establishment — LCP packets are exchanged to configure and test the basic link.
2. Authentication (optional) — if configured, either PAP or CHAP is used to authenticate the link.
3. Network layer protocol phase — NCP packets are exchanged to configure whichever Layer 3 protocol will actually use the link.

It’s worth noting that Cisco routers default to HDLC on serial interfaces, which is a Cisco-proprietary encapsulation. PPP has to be configured explicitly, and it is the natural choice whenever a Cisco router needs to connect to a non-Cisco router, since it’s an open standard rather than vendor-specific.