Cisco Cisco IOS Software Release 12.4(4)T

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In an Ethernet access network, there is no unique mapping between the subscriber line identifier and the 
interface such as there is on a virtual circuit (VC) in an ATM-based network. The PPPoE Circuit-Id Tag 
Processing feature provides a way to extract a Circuit-Id tag from the digital subscriber line (DSL) as an 
identifier for the authentication, authorization, and accounting (AAA) access request on an Ethernet 
interface, thereby simulating ATM-based broadband access, but using cost-effective Ethernet instead. 
The tag is useful for troubleshooting the network, and is also used in RADIUS authentication and 
accounting processes.

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12.4(4)T

This feature was introduced.