The Point-to-Point Protocol (PPP) is the most widely used protocol for connecting hosts over the peer-to-peer connection, such as modem, ISDN link or a direct cable connection. It supports transmitting multi-protocol datagrams over a single link, along with such features as authentication, link quality monitoring and flexible set of options which can be negotiated between two connecting hosts.

The PPP over Ethernet (PPPoE) protocol allows hosts inside an Ethernet-based network to access external hosts via PPP through the bridge host (Access Concentrator). It's most often used to access an xDSL line from inside the network, the DSL or cable modem playing the role of the AC.

Thus, for successful connection two physical parts are required: client PPPoE module (on every host wishing to use PPPoE) and one or more Access Concentrators. They're presented in a CPX by two modules: POECLI for client part, and POEAC as a server part, described in this chapter.

PPPoE Access Concentrators (AC) answer the PPPoE request coming from a client site PPPoE application. It offers standard PPP features such as authentication, encryption, and compression. It allows connection with login and password for Internet connection accounting purposes. Also, the IP address on the other side of the link is only assigned when the PPPoE connection is open, allowing the dynamic reuse of IP addresses.