Point-to-Point Protocol over Ethernet (PPPoE)
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
PPPOE OVERVIEW
PPP is typically the protocol of choice for an ISP offering IP services over DSL, as
PPP is the normal protocol for dialup access and most hosts have PPP client
functionality. PPP sessions can be carried over Ethernet as PPPoE, or over AAL5
as PPPoA.
When a corporate office has a DSL connection over a standard telephone line
from its ISP, there are three typical forms this connectivity can take at the
enterprise edge, if the ISP uses PPPoE.
• CPE router connects to a DSL router over Ethernet, and the DSL router takes in
the DSL line.
• CPE router connects to a DSL modem (or bridge) over Ethernet, and the DSL
modem takes in the DSL line.
• CPE router that interfaces to the WAN takes in a direct DSL line from the ISP.
PPPOE OPERATION
If a PPP session, which is essentially a point-to-point connection is to be
established over Ethernet, which is a broadcast network. It is necessary for each
peer to learn the MAC address of the remote PPP entity, so that the packets
belonging to the session can be directed to that entity.
This learning is accomplished by the PPPoE Discovery protocol. PPP is a more-
or-less symmetric protocol, though it allows asymmetric or client-server operation
such as the giving of an IP address to the peer. However, as the intent behind
PPPoE is to use it in a client-server scenario such as a corporate office using a
connection to its ISP, the PPPoE protocol describes a client-server relationship.
PPPoE clients attempt to discover a PPPoE server through an Ethernet broadcast
(the PADI packet). One or more PPPoE servers on the Ethernet LAN respond
through a PADO packet that contains the server's MAC address in the Ethernet
Source MAC field. The PPPoE client selects one of the servers, and
communicates this selection through a PADR packet to the server. As a PPPoE
server may accept multiple client sessions, it allocates a unique session ID, and
sends it to the client in a PADS packet. This ends the PPPoE Discovery Stage.
Now, the PPPoE Session Stage starts and the peers are ready to exchange LCP
packets and begin the PPP negotiation. Each LCP, authentication, IPCP or IP
packet on this session in either direction will have the PPPoE header, in addition
to the Ethernet header. The PPPoE header will contain the session ID that was
sent by the server in the PADS packet.
ALCATEL-LUCENT SPECIFIC OVERVIEW ON PPPOE FEATURES
The following features are available with the current release:
• Supports PPPoE on Gigabit Ethernet interfaces.
• The OmniAccess 5740 USG supports RFC 2516 (PPPoE) without necessarily
conforming to all the optional items mentioned in the specification.
• 1 PPPoE client session per SE Gigabit Ethernet interface.
• LCP (RFC 1661), IPCP (RFC 1332), PAP (RFC 1334), CHAP (RFC 1994), EAP
(RFC 3748) support.
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