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CHAPTER 3: SYSTEM CONFIGURATION
3.4.3 RELAY
A DHCP relay agent is used to forward and to transfer DHCP messages between the clients and the server when they are not in the
same subnet domain. It stores the incoming interface IP address in the GIADDR field of the DHCP packet. The DHCP server can use
the value of the GIADDR field to determine the assigned subnet. Be sure to configure the VLAN interface IP address and PVID (Port
VLAN ID) correctly.
WEB INTERFACE
To configure DHCP Relay in the web interface:
1. Click Configuration, DHCP, Relay.
2. Specify the Relay Mode, Relay server, Relay Information Mode, Relay Information.
3. Click Apply.
FIGURE 3-14. DHCP RELAY CONFIGURATION SCREEN
Parameter description:
Relay Mode: Indicates the DHCP relay mode operation. Possible modes are:
- Enabled: Enable DHCP relay mode operation. When DHCP relay mode operation is enabled, the agent forwards and transfers DHCP
messages between the clients and the server when they are not in the same subnet domain. And the DHCP broadcast message won’t be
flooded for security considerations.
- Disabled: Disable DHCP relay mode operation.
Relay Server: Indicates the DHCP relay server’s IP address.
Relay Information Mode: Indicates the DHCP relay information mode option operation. The option 82 circuit ID is formatted as
“[vlan_id][module_id][port_no]”. The first four characters represent the VLAN ID, the fifth and sixth characters are the module ID (in
a standalone device it always equal 0, in a stackable device it is the switch ID), and the last two characters are the port number. For
example, “00030108” means the DHCP message is received from VLAN ID 3, switch ID 1, port No 8. The Option 82 remote ID value
equals the switch MAC address. Possible modes are:
- Enabled: Enable DHCP relay information mode operation. When DHCP relay information mode operation is enabled, the agent inserts specific
information (option 82) into a DHCP message when forwarding to DHCP server and removes it from a DHCP message when transferring to
DHCP client. It only works when DHCP relay operation mode is enabled.
- Disabled: Disable DHCP relay information mode operation.
Relay Information Policy: Indicates the DHCP relay information option policy. When DHCP relay information mode operation is
enabled, if the agent receives a DHCP message that already contains relay agent information, it will enforce the policy. The ‘Replace’
policy is invalid when relay information mode is disabled. Possible policies are:
- Replace: Replace the original relay information when a DHCP message that already contains it is received.
- Keep: Keep the original relay information when a DHCP message that already contains it is received.
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