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TRENDnet User's Guide
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
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Router
Switch
DHCP client-2
DHCP Server
•
Application-2 (Local in different VLANs)
The DHCP cleint-1 and DHCP client-2 are located in different VLAN. But they allocate
IP address from the same DHCP server.
Server
Client
Client
Client
VLAN 1: port 1, 2 (Management VLAN)
VLAN 2: port 3, 4
VLAN 3: port 5, 6
VLAN 4: port 7, 8
DHCP Server ➔ Port 1.
DHCP Client ➔ Port 2, 3, 4, 5, 6, 7, 8.
Result: Hosts connected to port 2,3,4,5,6,7,8 can get IP from DHCP server.
Note:
The DHCP Server must connect to the management VLAN member ports.
The DHCP Relay in management VLAN should be enabled.
Default Settings:
The default global DHCP relay state is disabled.
The default VLAN DHCP relay state is disabled for all VLANs.
The default DHCP server is 0.0.0.0
Managed Industrial L2 Switch
Client
Client
Client
Client
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