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76
Enhanced Features
WAGO-ETHERNET-Zubehör 852
852-1305 8/4-Port 100BASE-T/1000BASE-SX/LX
Manual
1.1.0
7.2.4
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 is either a host or an IP router waits 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, but there are some
circumstances where unicast addresses are used. This can be the case when
networks have a single DHCP server that provides IP addresses for multiple
subnets. A router for such a subnet receives the DHCP broadcasts, converts them
to unicast (with a MAC/IP destination address of the configured DHCP server,
MAC/IP source address of the router itself). The GIADDR field on the main
DHCP page contains 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 selects 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
A user can enable/disable the “DHCP Relay” on the switch. It can also be
enabled/disabled on a specific VLAN. If the “DHCP Relay” on the switch is
disabled, it is disabled on all VLANs, even if enabled for individual VLANs.
Applications
•
Application 1 (via a router
)
DHCP client 1 and DHCP client 2 are in different IP segments. However,
they receive IP address from the same DHCP server.