Moxa Managed Ethernet Switch (UI_2.0_FW_5.x) User Manual
DHCP Relay Agent (Option 82)
Option 82 is used by the relay agent to insert additional inf ormation into the client’s DHCP request. The
Relay Agent Information option is inserted by the DHCP relay agent when f orwarding client-originated DHCP
packets to a DHCP server. Servers can recognize the Relay Agent Information option and use the
inf ormation to implement IP addresses to Clients.
When Option 82 is enabled on the switch, a subscriber device is identif ied 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 def ine 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 f ormat 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 f ormed by the port
VLAN ID in hex, and the last byte “PP” is f ormed 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-def ined string.
Server IP Address
1st Server
IP address for the 1st
Assigns the IP address of the 1st DHCP server that the switch
None
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