Dhcp Relay - Tripp Lite NGI-M08POE8-L2 Owner's Manual

6.5 DHCP Relay

Because the DHCPDISCOVER message is a broadcast message, and broadcasts only cross other
segments when they are explicitly routed, you might have to configure a DHCP Relay Agent on
the router interface so that all DHCPDISCOVER messages can be forwarded to your DHCP
server. Alternatively, you can configure the router to forward DHCP messages and BOOTP
message. In a routed network, you would need DHCP Relay Agents if you plan to implement only
one DHCP server.
The DHCP Relay that either a host or an IP router that listens for DHCP client messages being
broadcast on a subnet and then forwards those DHCP messages directly to a configured DHCP
server. The DHCP server sends DHCP response messages directly back to the DHCP relay agent,
which then forwards them to the DHCP client. The DHCP administrator uses DHCP relay agents
to centralize DHCP servers, avoiding the need for a DHCP server on each subnet.
Most of the time in small networks DHCP uses broadcasts however there are some circumstances
where unicast addresses will be used. A router for such a subnet receives the DHCP broadcasts,
converts them to unicast (with a destination MAC/IP address of the configured DHCP server,
source MAC/IP of the router itself). The field identified as the GIADDR in the main DHCP page
is populated with the IP address of the interface on the router it received the DHCP request on.
The DHCP server uses the GIADDR field to identify the subnet the device and select an IP
address from the correct pool. The DHCP server then sends the DHCP OFFER back to the router
via unicast which then converts it back to a broadcast and out to the correct subnet containing
the device requesting an address.
Configurations:
Users can enable/disable the DHCP Relay on the Switch. Users also can enable/disable the DHCP
Relay on a specific VLAN. If the DHCP Relay on the Switch is disabled, the DHCP Relay is
disabled on all VLANs even some of the VLAN DHCP Relay are enabled.
Applications
Application-1 (Over a Router)

The DHCP cleint-1 and DHCP client-2 are located in different IP segments. But they
allocate IP address from the same DHCP server.
DHCP Client-1 Switch
DHCP Server
Application-2 (Local in different VLANs)

Router Switch
150
DHCP Client-2