Single DSLAM Connected to a PPPoE Access Concentrator

This topic describes a sample configuration case to illustrate how the interworking functions are performed when a 1:1 association exists between DSLAM and the PPPoE access concentrator (B-RAS application).

Consider a topology in which a DSLAM multiplexes multiple PPPoE over ATM sessions over the Ethernet aggregate network using interworking functionality between PPPoE over ATM and PPPoE. Figure 1 shows a sample configuration scenario in which one DSLAM is connected to one PPPoE access concentrator. The subscriber access nodes are connected using ATM links to the DSLAMs. In this circuit, a 1:1 relationship exists between the DSLAM and the PPPoE access concentrator (PPPoE major interface). Because of the presence of a 1:1 association between the DSLAM and the access concentrator, segregation of VLAN IDs is not required. A one-to-one mapping is configured between the user port and VLAN. The access node is assigned a unique VLAN identification to a port using either a unique S-VLAN ID or a unique (S-VLAN ID, C-VLAN ID) pair. The uniqueness of the mapping is maintained in the access node and across the aggregation network. The B-RAS PPPoE access concentrator demultiplexes the PPPoE sessions.

Figure 1: Single DSLAM Connected to a PPPoE Access Concentrator

In such a case, the PPPoE access concentrator uses the maximum number of subinterfaces configured on a PPPoE interface using the pppoe sessions command to limit the maximum number of IWF PPPoE sessions. The B-RAS tracks the IWF flag in a PPPoE PADR packet it receives from the access concentrator, which the PPPoE client forwarded to the access concentrator during the discovery stage. The presence of the IWF tag causes the PPPoE access concentrator to accept duplicate PPPoE sessions with the same MAC address of the DSLAM are permitted by default. The suboption for the IWF-Session DSL Forum VSA (26-254) contains the field code of 0xFE in PADR packet and its length field is set to 0x00.