User`s manual

TN-5516/5518 Series Featured Functions
3-67
Configuring DHCP Relay Agent
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
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 recognize the Relay Agent Information option and use the Information to implement
IP address assignment policies to the Client.
When Option 82 is enabled on the switch, a subscriber device or host 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
combining the Ethernet switch’s physical port number and VLAN ID. The format of the “Circuit ID” is 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. It can be one of the following types:
1. The IP address of the relay agent.
2. The MAC address of the relay agent.
3. The combination of IP address and MAC address of the relay agent.
4. A user-defined string.