Pppoe Stages; Discovery; Figure 37: Pppoe Over Atm - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - LINK LAYER CONFIGURATION GUIDE 2010-10-13 Configuration Manual

JunosE 11.3.x Link Layer Configuration Guide

PPPoE Stages

372
Remote Access Server (B-RAS) technologies that provide a bridged Ethernet topology.
PPPoE can be configured over ATM or on Ethernet modules with or without VLANs.
Figure 37 on page 372 shows how PPPoE allows the router to handle multiple PPP sessions
originating on an Ethernet module to be multiplexed over one PVC on an ATM interface.
PPP, as described in "Configuring Point-to-Point Protocol" on page 259, runs above the
PPPoE layer.

Figure 37: PPPoE over ATM

The router handles the server part of PPPoE session management and never initiates a
setup of a PPPoE session. The router only responds to session requests that are sent to
it by the remote PPP client. After the sessions are set up, the router demultiplexes the
sessions based on session identifiers assigned to a specific connection.
PPPoE has two distinct stages: Discovery and Session.

Discovery

PPPoE includes a Discovery protocol that allows each PPP session to learn the Ethernet
address of the remote peer, as well as establish a unique session identifier. When a host
wants to initiate a PPPoE session, it must first perform Discovery to identify the Ethernet
MAC address of the peer and establish a PPPoE session ID.
Although PPP defines a peer-to-peer relationship, Discovery is inherently a client-server
relationship. In the Discovery process, a host acting as a client discovers a remote access
concentrator (AC), which acts as the server.
Based on the network topology, there may be more than one remote AC with whom the
host can communicate. The Discovery stage allows the host to discover all remote ACs
and then select the one to which it wants to connect.
In summary, the Discovery stage consists of the following four steps:
The host (PPPoE client) broadcasts a PPPoE Active Discovery Initiation (PADI) packet
1.
to all remote ACs in the network.
One or more remote ACs respond to the PADI packet by sending a PPPoE Active
2.
Discovery Offer (PADO) packet, indicating that they can serve the client request. The
PADO packet includes the name of the AC from which it was sent.
Copyright © 2010, Juniper Networks, Inc.