FUJITSU PSWITCH User’s Guide
100 December/2018
3.1.4.3. DHCP Relay
The DHCP relay agent is a network device used for communication between DHCP
client and server when they are placed in different subnets and to complete the
DHCP protocol operation.
IPv4
Use IP Helper to implement DHCPv4 relay. Please refer to “3.1.9.19. UDP Relay/IP
Helper” for detail description.
IPv6
DHCPv6 relay agents listen for the DHCPv6 messages that are broadcast on the
local subnet and route them to a DHCPv6 server on a different subnet.
3.1.4.4. DHCP L2 Relay
This feature only supported IPv4. Such a device, a L3 relay agent, is generally a
router that has IP interfaces on both the client and server subnets and can route
between them. However in a L2 switched networks there may be one or more
infrastructure devices (such as a switch) in between the client and the L3 relay
agent; and so some of the client device information required by the L3 relay agent
may not be visible to it. In this case, a L2 Relay agent can be used to add the
further information that the L3 relay agent and DHCP server may need to complete
the task required by the network administrator.
The DHCP relay agent's role was expanded by RFC 3046, which specifies the DHCP
relay agent information option, more commonly referred to as DHCP option-82.
The relay agent may add this option to packets it relays from clients to servers, and
must remove this option from packets it relays from servers to clients. The relay
agent information option defines two sub-options with defined meanings,
circuit-id and remote-id, to carry information that the relay agent may know about
the circuit or attachment point of the client, and about the client's identity. The
format of these sub-options is implementation-specific. These sub-options may be
used by the DHCP server to affect how it treats the client, and also may be used by
the relay agent to limit broadcast replies to the specific circuit or attachment point
of the client. The circuit-id added is the port number for the client requests. The
configuration of the options is based on the service VLAN ID.
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