The circuit ID and remote ID fields (option 82) are part of the DHCP relay agent information option. A digital subscriber line access multiplexer (DSLAM) inserts the option 82 fields into DHCP messages; the customer appliance inserts the option 60 fields.
You can configure an ISG policy to use the circuit ID, remote ID, or vendor class ID, or a combination of the three, as the username in authorization requests. Alternatively, you can configure an ISG policy to use the NAS-Port-ID as the identifier for authorization. When you use the NAS-Port-ID as the identifier, you can configure it to include a combination of circuit ID, remote ID, and vendor-class ID.
By default, the ISG uses the circuit ID and remote ID that are provided by the Layer 2 edge-access device for authorization. The configuration of the ip dhcp relay information option command determines whether the ISG uses the option 82 information received, generates its own, or (when the encapsulate keyword is specified) encapsulates a prior option 82 along with its own option 82. For more information, see the "Configuring the Cisco IOS DHCP Relay Agent" section of the Cisco IOS IP Addressing Services Configuration Guide .
If the NAS-Port-ID is not configured to include option 60 and option 82, the NAS-Port-ID is populated with the ISG interface that received the DHCP relay agent information packet; for example, Ethernet1/0.