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    Understanding PPPoE

    E Series routers use PPP over Ethernet (PPPoE) to enable multiple hosts to open PPP sessions to the router using one or more bridging modems. When service providers want to maintain the session abstraction associated with PPP, PPPoE is used with Broadband 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 1 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 Understanding PPP, runs above the PPPoE layer.

    Figure 1: PPPoE over ATM

    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 Stages

    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:

    1. The host (PPPoE client) broadcasts a PPPoE Active Discovery Initiation (PADI) packet to all remote ACs in the network.
    2. One or more remote ACs respond to the PADI packet by sending a PPPoE Active 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.
    3. The host sends a unicast PPPoE Active Discovery Request (PADR) packet to the AC to which it wants to connect.
    4. The selected AC sends a PPPoE Active Discovery Session (PADS) packet to confirm the session.

    Session

    When Discovery is successfully completed, both the host and the selected remote AC have the information they need to build their point-to-point connection over Ethernet.

    The only parameter that you can configure is the number of PPPoE sessions.

    Note: The router supports dynamic PPPoE interfaces. Also, profiles support PPPoE interfaces. See Dynamic Interfaces Overview and Configuring Dynamic Interfaces Using Bulk Configuration Overview, for more information.

    PPPoE MTU Configuration

    To avoid fragmentation and reassembly, Ethernet access networks require larger MTU sizes for PPP traffic. With JunosE PPPoE MTU, you can control the deployment of larger packet sizes. You can configure PPPoE MTU directly on the PPPoE interface or use a dynamic configuration profile. When you use the PPPoE MTU tag, each PPPoE subinterface can have a unique MTU value. Operational MTU is the lesser of the PPPoE MTU or the lower layer MTU minus the PPPoE overhead.

    You can use the pppoe mtu command to set the MTU using a combination of lower layer restrictions and controls:

    • Greater MTU than the current maximum permitted by RFC 2516, with the default equal to the current maximum setting (1494 octets)
    • Optional setting for absolute maximum PPPoE MTU
    • Optional use of a larger lower layer MTU
    • Optional use of the PPPoE-Max-Mtu tag transmitted from the client

    Published: 2014-08-14