PLANET XGS3-24042 - Manual (Page 566)

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Chapter 61 PPPoE Intermediate Agent
Configuration
61.1 Introduction to PPPoE Intermediate Agent
61.1.1 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.
61.1.2 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:
1. 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.
2. Broadband Access Server responds PADO packet: The second step, server responds PADO
(PPPoE Active Discovery Offer) packet to client according to the received source MAC address of
PADI packet, the packet will take sever name and service name.
3. Client sends PADR packet: The third step, client selects a server to process the session according
to the received PADO packet. It may receives many PADO packets for PADI message of the first
step may be sent to many servers (select the server according to whether the service information of