In a prior lab, Setting-up DHCP on Cisco Routers, I discussed how to setup a Cisco router as a DHCP server. In this lab, I use a DHCP server setup on one subnet to provide DHCP services to two different subnets. For this lab I use a Cisco 2126XM router [IOS Version 12.2(28)] as the layer 3 device and two Cisco WS-C2950-24 switches [IOS Version 12.1(22)EA4X] as the layer 2 devices. The network diagram and DHCP server configuration for this lab is shown below.
Network diagram for Cisco "ip helper-address" lab
Notice on the below DHCP configuration I have DHCP setup for two different subnets, 192.168.1.0/24 and 10.0.0.0/24.
DHCP server configuration
The general issue we’re addressing here is that although routers accept and generate broadcasts, they do not forward them. This becomes problematic when a broadcast needs to get to a DHCP server, DNS server, TFTP server, or some other device that is on a different subnet connected to the router. If a PC tries to locate a device such as a DHCP server via broadcast, the router will prevent the broadcast from reaching the DHCP server on the different subnet.
By using the “ip helper-address” command on a router interface that will be receiving the broadcasts, UDP broadcasts such as DHCP broadcasts will be translated into unicast by the router. This will enable proper communication with the DHCP server on the different subnet. See below for the configuration used to enable the PCs on the 10.0.0.0/24 network to communicate with the DHCP server on the 192.168.1.0/24 network. Note that “ip helper-address” configuration is not needed if DHCP is only required for PCs on the same subnet.
"ip helper-address" configuration
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