User Documentation

User Manual Managed Switches
116
Desired IP Address
Setting
Description
Factory
Default
IP Address
Set the desired IP of connected devices.
None
3.12.2 DHCP Relay Agent (Option 82)
The DHCP Relay Agent makes it possible for DHCP broadcast messages to be sent over routers.
The DHCP Relay Agent enables DHCP clients to obtain IP addresses from a DHCP server on a
remote subnet, or those that are not located on the local subnet.
DHCP 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 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 as described below:
FF–VV–VV–PP
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" is to identify the relay agent itself and it can be one of the following:
 The IP address of the relay agent.
 The MAC address of the relay agent.
 A combination of IP address and MAC address of the relay agent.
 A user-defined string.