Traditional ATM-based DSL broadband access networks have the topology
shown in The figure below.
Figure 1. ATM-Based DSL Broadband Access Network
In terms of logical connectivity, there is a one-to-one mapping of the
DSL subscriber line to the end user and the ATM virtual circuit (VC) used to
carry the PPP session through the DSLAM and to the BRAS, where this VC
information is converted into a NAS-Port-ID tag for use in RADIUS packets.
The simple mapping available from an ATM-based broadband network
between the physical line in the DSL local loop to the end user and a virtual
circuit (from DSLAM to BRAS) is not available for a Fast or Gigabit
Ethernet-based network. To solve this problem, the PPPoE Remote-ID Tag
Processing feature uses a PPPoE intermediate agent function on the DSLAM to
attach a tag to the PPPoE discovery packets. The BRAS then receives the tagged
packet, decodes the tag, and inserts the line identifier into RADIUS packets
destined for the RADIUS server.
The DSLAM intercepts PPPoE discovery frames from the client or
initiates a discovery frame if the PPPoE Active Discovery (PAD) client is a
legacy PPP over ATM (PPPoA) device. The DSLAM inserts a unique Remote-ID tag
and DSL sync rate tag using the PPPoE vendor-specific tag (0x0105) to PPPoE
Active Discovery Initiation (PADI) and PPPoE Active Discovery Request (PADR)
packets; see the figure below. The DSLAM forwards these packets upstream to the
BRAS after the insertion. The tag contains the identification of the DSL line
on which the PADI or PADR packet was received, in the access node where the
intermediate agent resides.
Figure 2. PPPoE Remote-ID Tag Processing Solution
vendor-tag remote-id service command is configured in broadband access
(BBA) group configuration mode, the BRAS processes the received PPPoE
vendor-specific tag in the PADR frame and extracts the Remote-ID tag, which is
sent to the remote AAA server as a VSA in all AAA access and accounting
requests. When the
radius-server attribute 31 remote-idglobal configuration command is also configured on the BRAS, the
Remote-ID value is inserted into attribute 31.
Outgoing PAD Offer (PADO) and PAD Session-Confirmation (PADS) packets
from the BRAS have the DSLAM-inserted Remote-ID tag. The DSLAM should strip the
tag out of PADO and PADS frames. If the DSLAM cannot strip off the tag, the
BRAS must remove the tag before sending the frames out. This is accomplished
vendor-tag strip BBA group configuration mode command. If this
command is configured under the BBA group, the BRAS strips the incoming
Remote-ID tag (and any other vendor tag) off of the outgoing PADO and PADS
frames. This action complies with DSL Forum Technical Report 101 .