DHCP Option 82 provides the DHCP server with information about which switch and which port on that switch a DHCP request is coming from. This information is supplied via Agent-ID and Circuit-ID subfields of the Relay-Information DHCP Option, as defined in RFC 3046. DHCP snooping is Option-82 friendly in the sense that it can insert or remove DHCP relay information (Option-82 field) in forwarded DHCP request messages from untrusted ports to the DHCP server.
With Option 82 enabled, the DHCP server can use the extra information to assign IP addresses, perform access control, and set quality of service (QoS) and security policies (or other parameter-assignment policies) for each DHCP client. When the server returns a response, it also includes Option-82 information. Not all DHCP servers support Option 82, however. At the time of this writing, a Google search for "DHCP server option 82" returned just a few hits, among which Cisco Network Registrar and Avaya's server figured. Moreover, the DHCP server developed by Internet Systems Consortium (ISC) can log Option 82, which is called agent.circuit-id.
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