Point-to-point Protocol Over Ethernet - How PPPoE Fits in The DSL Internet Access Architecture - PPPoEoA

PPPoEoA

The diagram depicts one of two possible scenarios involving the use of PPPoE to connect an Ethernet router to a DSL modem (both PPPoE-speaking) in order to connect a LAN to a PPPoE-speaking ISP. Here three devices need to speak PPPoE; router, modem and a Broadband Remote Access Server located at the service provider or ISP's office. On the diagram, the deep protocol stack we see between modem and DSLAM is a picture of what we have termed ‘’PPPoEoA in this article. (Note that other authors, such as some Cisco publications also use the term ‘PPPoEoA’ but not necessarily with exactly the same meaning.)

Example: The DSL modem receives an Internet-bound Ethernet frame from the router which contains a PPP frame inside a PPPoE packet. The modem sends on the PPPoE packet toward the DSLAM by wrapping it in several protocol layers as follows: Typically in this PPPoEoA topology, the DSL modem wraps the PPPoE packet within Ethernet frame-wrapping as described in RFC 2684, second part, ‘bridged protocols’, before sending it on to the DSLAM within an AAL5-encapsulated packet over ATM.

This Ethernet frame wrapping around the PPPoE packets represents a significant and quite unnecessary overhead and can be avoided by using the alternate scheme discussed in the next section.

DSL Internet access architecture

Router		DSL modem		DSLAM		Remote access server		(ISP)
(IP)								(IP)
Ethernet	PPP	PPP	PPP			PPP	PPP	PPP
	PPPoE	PPPoE	PPPoE			PPPoE	backbone	backbone
	Ethernet	Ethernet	Ethernet			Ethernet	IP	IP
			RFC2684	RFC2684		backbone
			AAL5	AAL5	backbone	backbone
			ATM	ATM
			DSL	DSL

Notes:

• Where the diagram shows ‘(IP)’ this could be various kinds of PPP payload, IPv4 and IPv6 being just examples, but the payload need by no means be in any way IP-related. It can be anything that PPP knows how to encapsulate.
• On the diagram, the area shown as ‘backbone’ could also be ATM on older networks, however its architecture is service-provider dependant. There would be additional columns in this area in a more detailed service-provider specific diagram.
• The diagram does not show the whole of the ISP-side. The PPP connection typically terminates at a server further off the right hand side, not shown. The right hand portion of the diagram shown as backbone would typically be PPP/L2TP/IP.
• Where the diagram says ‘router’, a PPPoE-speaking single host PC could be an alternative to an Ethernet router, acting as a PPPoE client and PPP endpoint.