SGS-6341-Series User Manual

Chapter 51 PPPoE Intermediate Agent
Configuration
51.1 Introduction to PPPoE Intermediate Agent
51.1.1 Brief Introduction to PPPoE
PPPoE (Point to Point Protocol over Ethernet) is a protocol that applies PPP protocol to
Ethernet. PPP protocol is a link layer protocol that utilizes the 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 for the broadband access
authentication of Ethernet usually.
51.1.2 Introduction to PPPoE IA
Broadband access technique is rapidly developed. Broadband access network is also
developing from strength to strength, but security problem gradually becomes the focus, so 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 cannot 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 uses user accounts to login, this method supplied by PPPoE Intermediate Agent can avoid
the embezzlement of user accounts.
There are two stages for PPPoE protocol to 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:
1. Client sends PADI packet: For 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. Note: This message may be sent to
many access collectors of the network.
2. Broadband Access Server responds to PADO packet: For the second step, server
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User’s Manual of SGS-6341 series