Here is brief information about ip helper-address. If your DHCP server is located remotely, your local DHCP client might not get IP address due to broadcasting traffic is blocked by router.
By default, routers drop all broadcast packets sent through them. Because DHCP clients use BOOTP packets, which are broadcasted to all hosts (255.255.255.255), they will be dropped by router. The “ip helper-address” command enables the router to forward these BOOTP broadcast packets to a specific host, as specified by the address following the “ip helper-address” command. Note that this command must be placed on the router’s interface that is receiving the broadcast packets from the hosts, which is Ethernet(FastEthernet or GigabitEthernet Interface) of the router.
To see more detailed about IP helper-address with DHCP Server testing scenario on TCP/IP > DHCP on main menu option or click below article.
"How to use IP helper-address to connect remote DHCP server"
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