PPPoE
When PPPoE is used, a PPP session is established, similar to when using
dialup. Either Password Authentication Protocol (PAP) or Challenge
Handshake Authentication Protocol (CHAP) authentication can be used. The
provider’s aggregation router and the subscriber’s CPE establish a session
between them. There are three ways to do this:
■ The PPP client on a subscriber router with a DSL interface terminates
both the DSL and the PPP sessions. The router can allow multiple
users over the DSL with just one PPP login, by acting as a DHCP
server and doing Network Address Translation (NAT) or Port Address
Translation (PAT) for the subscriber users. The router obtains its
outside IP address via PPP’s IP Control Protocol (IPCP.)
■ A DSL modem terminates the DSL session, and the PPP client on a
CPE router terminates the PPP session. The router can act as a DHCP
server and do NAT/PAT, to allow multiple internal users. It obtains its
outside address via IPCP.
■ A DSL modem terminates the DSL session, and a PPP client on the
subscriber PC terminates the PPP session. Traffic is bridged from the
PC to the aggregation router. This allows only a single DSL user. If
multiple users are at the same residence, they must each have their
own PPP login, and they each obtain an IP address via IPCP.
PPP was created to be used over a point-to-point connection, and Ethernet is
inherently multipoint, so PPPoE uses a PPP server discovery process. After
a server has been discovered, a virtual point-to-point link can be established,
and the PPP session process can continue. The PPP server discovery stage
has four steps:
Step 1. The PPP client sends a PPPoE Active Discovery Initiation
(PADI) broadcast.
Step 2. Any PPP servers (aggregation routers) reply with a PPPoE Active
Discovery Offer (PADO), sent as a unicast to the client’s MAC
address.
Step 3. The client replies to the server with a PPPoE Active Discovery
Request (PADR).
Step 4. The server confirms the association with a PPPoE Active
Discovery Session-confirmation (PADS) message.
When these steps have been completed, the normal PPP session negotiations
proceed, and a session is established.
PPPoA
PPPoA requires a CPE router; traffic is routed from the subscriber PCs to
the aggregation router—it cannot be bridged as with PPPoE. The PPP
session is established between the CPE router and the aggregation router.
Multiple users are supported if the CPE router is configured to do DHCP
and NAT. Traffic between the CPE router and the aggregation router is
encapsulated as ATM at Layer 2, rather than Ethernet. Therefore, the CPE
router must have an ATM interface.