Reference Guide

254 | Dynamic Host Configuration Protocol (DHCP)
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Configure the System to be a Relay Agent
The following feature is available on platforms: c e s
DHCP clients and servers request and offer configuration information via broadcast DHCP messages.
Routers do not forward broadcasts, so if there are no DHCP servers on the subnet, the client does not
receive a response to its request and therefore cannot access the network.
You can configure an interface on the Dell Force10 system to relay the DHCP messages to a specific
DHCP server using the command
ip helper-address dhcp-address from INTERFACE mode, as shown in the
illustration below. Specify multiple DHCP servers by entering the
ip helper-address dhcp-address command
multiple times.
When
ip helper-address is configured, the system listens for DHCP broadcast messages on port 67. The
system rewrites packets received from the client and forwards it via unicast; the system rewrites the
destination IP address and writes its own address as the relay device. Responses from the server are unicast
back to the relay agent on port 68 and the relay agent rewrites the destination address and forwards the
packet to the client subnet via broadcast.
HCP Relay Device
Note: DHCP Relay is not available on Layer 2 interfaces and VLANs.
DHCP 001
R1(conf-if-gi-1/3)#show config
!
interface GigabitEthernet 1/3
ip address 10.11.0.3/24
ip helper-address 10.11.1.5
ip helper-address 10.11.2.5
no shutdown
1/3
Unicast
DHCP Server
10.11.1.5
DHCP Server
10.11.2.5
Broadcast
Source IP : 10.11.1.5
Destination IP: 255.255.255.255
Source Port: 67
Destination Port: 68
Unicast
Source IP : 10.11.1.3
Destination IP: 10.11.1.5
Source Port: 67
Destination Port: 67
Relay Agent Address: 10.11.0.3
1/4
Unicast
Source IP : 10.11.1.5
Destination IP: 10.11.0.3
Source Port: 67
Destination Port: 68
Broadcast
Source IP : 0.0.0.0
Destination IP: 255.255.255.255
Source Port: 68
Destination Port: 67
Relay Agent Address: 0.0.0.0