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ADSL WAN Connections
PPPoE Overview
Figure 7-9. Discovery Stage for Negotiating a PPPoE Session
Step 1. The PPPoE client broadcasts a PPPoE Active Discovery Initiation
(PADI) frame to locate the available access concentrators. This frame con-
tains at least one service name tag, which specifies the service that the PPPoE
client is requesting. As outlined in RFC 2516, the PADI frame (including the
PPPoE header) cannot be larger than 1484 bytes.
Step 2. The available access concentrators that can provide the service (or
services) specified in the PADI frame send a PPPoE Active Discovery Offer
(PADO) frame to the Ethernet MAC address of the PPPoE client. This frame
contains the name of the access concentrator and the service name tag that
was included in the PADI frame from the PPPoE client. In addition, the PADO
frame may include information about other services available from the access
concentrator.
Step 3. If the PPPoE client receives a PADO frame from more than one
access concentrator, it reviews the offers and selects one, based on either
name or services offered. For example, the PPPoE client may be configured
to accept the offer from a particular access concentrator. In this case, the
client makes the selection based on access concentrator name. Alternatively,
the PPPoE client may be configured to accept the offer based on the services
offered.
After making the selection, the PPPoE client sends a unicast frame called a
PPPoE Active Discovery Request (PADR) to the MAC address of the access
concentrator it selected. This frame contains the service name tag of the
service the PPPoE client is requesting.
1. PPPoE client broadcasts a PADI (initiation) frame
2. Access concentrator sends a PADO (offer) frame
3. PPPoE client sends a PADR (request) frame
4. Access concentrator sends a PADS (confirmation) frame
Router
Access
concentrator
Discovery Stage
If negotiation is successful, PPP session begins
Goal: Learn session ID and peer’s Ethernet MAC address
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