User Manual Managed Switches
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3.5.2 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 whilst 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.
3.5.2.1 DHCP Relay Configuration
This page configures DHCP Relay operation mode.
Relay Mode
Setting
Description
Factory Default
Enabled /
Disabled
Indicates the DHCP relay mode operation.
Enabled: Activates DHCP relay. When DHCP relay is
enabled, the agent forwards and transfers DHCP
messages between the clients and the server when
they are not in the same subnet domain to prevent the
DHCP broadcast message from flooding for security
considerations.
Disabled: Disables DHCP relay.
Disabled
Relay Server
Setting
Description
Factory Default
IP address
Indicates the DHCP relay server IP address. A DHCP
relay agent is used to forward and transfer DHCP
messages between the clients and the server when
they are not in the same subnet domain.
0.0.0.0
