Dell EMC Configuration Guide for the S3100 Series 9.14.2.4

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 EMC Networking system to relay the DHCP messages to a specific DHCP server using the ip helper-address dhcp-address command from INTERFACE mode, as shown in the following illustration. Specify multiple DHCP servers by using the ip helper-address dhcp-address command multiple times.

When you configure the ip helper-address command, the system listens for DHCP broadcast messages on port 67. The system rewrites packets received from the client and forwards them via unicast to the DHCP servers; 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 67 and the relay agent rewrites the destination address and forwards the packet to the client subnet via broadcast or unicast, depending whether the client has set or cleared the BROADCAST flag in the DHCP Client PDUs.

To view the ip helper-address configuration for an interface, use the show ip interface command from EXEC privilege mode.

R1_E600#show ip int gigabitethernet 1/3
GigabitEthernet 1/3 is up, line protocol is down
Internet address is 10.11.0.1/24
Broadcast address is 10.11.0.255
Address determined by user input
IP MTU is 1500 bytes
Helper address is 192.168.0.1
                  192.168.0.2
Directed broadcast forwarding is disabled
Proxy ARP is enabled
Split Horizon is enabled
Poison Reverse is disabled
ICMP redirects are not sent
ICMP unreachables are not sent