Point-to-Point Protocol over Ethernet (PPPoE) is, obviously, a twist on traditional PPP implementations. It is essentially a bridging architecture. Typical bridging implementations include wide-ranging security holes. Adding PPP architecture (using PAP or CHAP authentication) on top of this Ethernet bridging function alleviates the security holes and provides a well-known, robust platform.
PPPoE, as defined in RFC 2516, provides the ability to connect a network of hosts over a simple bridging access device to a remote access concentrator, or in this discussion, an aggregation router. Figure 4-5 shows the connectivity between the subscriber host and the aggregation router.
The DSLAM terminates the Layer 1 DSL connection and pushes the payload out the other side to ride the chosen media type (copper/fiber, and so on) across the ATM network. From CPE router to aggregation router, the only OSI layers used are Layers 1 and 2. The first Layer 3 function occurs once PPP negotiation has completed between the CPE and the aggregation router.
It should be pointed out that either a DSL-capable CPE router or a subscriber PC running PPPoE-capable client software may provide the subscriber side of the PPPoE connection. In either event, PPP frames are encapsulated inside of Ethernet frames for transport across the network. IP address allocation is handled by a provider DHCP server once the IPCP portion of the PPP connection is established.
Figure 4-5 PPPoE Topology
With this model, each router uses its own PPP stack and the user is presented with a familiar user interface. Access control, billing, and provision of service can be performed on a per-user, rather than a per-site, basis.
To provide point-to-point connections over Ethernet, each PPP session must learn the MAC address of the remote peer and establish a unique session identifier. PPPoE includes a discovery protocol that provides this function.
As with traditional dialup PPP sessions, the link must be created and initialized. The PPPoE initialization process has added two additional phases:
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