Option 82 is used by the relay agent to insert additional information into the client’s DHCP request. The Relay Agent Information option is inserted by the DHCP relay agent when forwarding client-originated DHCP packets to a DHCP server. Servers can recognize the Relay Agent Information option and use the information to implement IP addresses to Clients.
When Option 82 is enabled on the switch, a subscriber device is identified by the switch port through which it connects to the network (in addition to its MAC address). Multiple hosts on the subscriber LAN can be connected to the same port on the access switch and are uniquely identified.
The Option 82 information contains 2 sub-options, Circuit ID and Remote ID, which define the relationship between the end device IP and the DHCP Option 82 server. The Circuit ID is a 4-byte number generated by the Ethernet switch—a combination of physical port number and VLAN ID. The format of the Circuit ID is shown below:
FF–VV–VV–PP
This is where the first byte “FF” is fixed to “01”, the second and the third byte “VV-VV” is formed by the port VLAN ID in hex, and the last byte “PP” is formed by the port number in hex. For example:
01–00–0F–03 is the “Circuit ID” of port number 3 with port VLAN ID 15.
The “Remote ID” identifies the relay agent itself and can be one of the following:
1. The IP address of the relay agent.
2. The MAC address of the relay agent.
3. A combination of IP address and MAC address of the relay agent.
4. A user-defined string.
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