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Chapter 47 PPPoE Intermediate Agent
Configuration
47.1 Introduction to PPPoE
Intermediate Agent
47.1.1 Brief Introduction to PPPoE
PPPoE (Point to Point Protocol over Ethernet) is a protocol that apply PPP protocol to
Ethernet. PPP protocol is a link layer protocol and supply a communication method of point-to-
point, it is usually selected by host dial-up link, for example the link is line dial-up. PPP protocol
is applied to Ethernet that means PPPoE protocol makes many hosts of Ethernet to connect a
remote access collector through one or multiple bridge devices. If the remote access collector
is broadband access server (BAS), it can supply broadband access and accounting functions
for these hosts, so PPPoE protocol is used to broadband access authentication of Ethernet
usually.
47.1.2 Introduction to PPPoE IA
Along with broadband access technique is rapidly developed, broadband access network is
also developing from strength to strength, but security problem gradually becomes the focus,
soever the clients or the access device and the network are faced with security problem
(especially from the client) in the current access network. Traditional Ethernet user can not be
identified, traced and located exactly, however in exoteric and controllable network,
identification and location are the basic character and requirement for user, for example, when
supplying the application that use user accounts to login, this method supplied by PPPoE
Intermediate Agent can availably avoid user accounts embezzled.
There are two stages for PPPoE protocol work: discovery stage and session stage. Discovery
stage is used to obtain MAC address of the remote server to establish a point-to-point link and
a session ID with the server, and session stage uses this session ID to communicate. PPPoE
Intermediate Agent only relates to discovery stage, so we simply introduce discovery stage.
There are four steps for discovery stage:
Client sends PADI packet: The first step, client uses broadcast address as destination address
and broadcast PADI (PPPoE Active Discovery Initiation) packet to discover access collector in
layer 2 network. Notice: This message may be sent to many access collector of the network.
Broadband Access Server responds PADO packet: The second step, server responds PADO