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 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 for use in RADIUS packets.
The simple mapping available from an ATM-based network between the
physical line in the DSL local loop to the end user and a VC (from DSLAM to
BRAS) is not available for an Fast or Gigabit Ethernet-based network. To solve
this problem, the PPPoE Circuit-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.
DSLAM intercepts PPPoE discovery frames from the client and inserts a
unique line identifier (circuit-id) using the PPPoE Vendor-Specific tag
(0x0105) to PPPoE Active Discovery Initiation and Request (PADI and PADR)
packets; see the figure below. The DSLAM forwards these packets to the BRAS
after the insertion. The tag contains the circuit-id of the DSL line on which
the PADI or PADR packet was received, in the access node where the intermediate
Figure 2. PPPoE Circuit-Id Tag Processing Solution
vendor-tag circuit-id service command is configured in BBA (broadband
access) group configuration mode, the BRAS processes the received PPPoE
Vendor-Specific tag in the PADR packet and extracts the Circuit-Id field, which
is sent to the remote AAA server as the NAS-Port-Id attribute (RADIUS attribute
87) in RADIUS access and accounting requests. When the
radius-server attribute nas-port format d global configuration command is also configured on
the BRAS, the Acct-Session-Id attribute will contain the information about the
incoming access interface, where discovery frames are received, and about the
session being established.
Outgoing PAD Offer and Session-confirmation (PADO and PADS) packets
from the BRAS will have the DSLAM-inserted Circuit-Id tag. DSLAM should strip
the tag out of PADO and PADS packets. If the DSLAM cannot strip off the tag,
the BRAS should remove it before sending the packets out, and this is
accomplished using the
vendor-tag circuit-id strip BBA group configuration mode command.