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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 Router Switch DHCP Client-2