Encapsulation Type Lockout for IWF PPPoE Sessions Overview

Consider a sample configuration scenario in which subscriber access loops are connected using ATM links to a digital subscriber line access multiplexer (DSLAM) device. Figure 1 shows a topology in which one DSLAM is connected one PPPoE access concentrator. The ATM traffic is forwarded using an Ethernet circuit from the DSLAM device to the PPPoE access concentrator. The PPPoE over ATM or PPP over ATM (PPPoA) traffic is converted to PPPoE by the DSLAM device, which is connected to an ATM access loop on one side and an Ethernet aggregate network on the other side. The interworking function (IWF) describes the set of mechanisms used to convert a PPPoE over ATM or PPPoA session to a PPPoE session.

Figure 1: Single DSLAM Connected to a PPPoE Access Concentrator

The conversion of PPP, PPPoE, Ethernet, ATM 1483, or ATM connections at the DSLAM access loop interface to PPP, PPPoE, or Ethernet connections at the DSLAM aggregate network interface signifies that the MAC addresses of all such IWF sessions are translated to be the MAC address of the DSLAM Ethernet interface connected to the aggregate network. Therefore, you cannot uniquely identify a subscriber using the media access control (MAC) address only. If the MAC address alone is used to identify a particular subscriber, a single erroneous IWF session can cause other IWF sessions to be locked out.

In this setup, if the PPPoE session contains the IWF-Session DSL Forum VSA (26-254) in the PPPoE active discovery request (PADR) packet that is sent from the PPPoE client, the Agent-Circuit-Id DSL Forum VSA (26-1) is used in addition to the MAC address to identify the PPPoE session to be locked out. This method enables backward compatibility with all non-IWF topologies. The Agent-Circuit-Id is of the format, DSLAM name Slot/Port VPI: VCI, which enables unique identification of the subscriber that has initiated the session. This feature of encapsulation type lockout based on the Agent-Circuit-Id, in addition to the MAC address retrieved from the client, is enabled by default only for IWF sessions.