Cisco Cisco IOS Software Release 12.4(4)T

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 virtual circuit (from DSLAM to BRAS) is not available for an 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 using the 
PPPoE Vendor-Specific tag (0x0105) to PADI and PADR (PPPoE Active Discovery Initiation and 
Request) packets; see 

. The DSLAM forwards these packets to the BRAS after the insertion. The 

tag contains the Line-Id of the DSL line on which the PADI or PADR packet was received, in the access 
node where the intermediate agent resides.

When the 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 PADO and PADS (PAD Offer and Session-confirmation) 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.

DSLAM

Subscriber

lines

ATM

network

BRAS

To IP network

and RADIUS

server

142354

DSLAM

Subscriber

lines

Ethernet

connectivity from
DSLAM to BRAS

BRAS

To IP network

and RADIUS

server

142355

DSLAM intercepts discovery packets and inserts a unique PPPoE Line-ID 
tag to PADI or PADR packets and forwards upstream to BRAS.

BRAS processes the tag and extracts the Remote-ID, which is stored on 
the session.

The Remote-ID is sent as a NAS-Port-ID attribute in AAA accounting and 
PPP authentication requests.