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There are times when you don’t want to use the routers built in DHCP server and want to use say a Windows Server to hand out your networks DHCP scopes. This guide is for basic DHCP relay configuration in Cisco IOS.

In this example we are going to be configuring the “ip helper” command on interface FastEthernet0/1. Our IP for the interface will be 10.10.20.1/24 and the IP of our DHCP server will be 10.10.10.5/24.

After you have accessed the command line of the router the first thing you will need to do is get into configuration mode by issuing the following command:

conf t

The DHCP server and relay agent should be enabled by default, however if for some reason they are not already enabled they can be enabled by issuing the following command:

service dhcp

Next we will want to enable the DHCP relay agent information option:

ip dhcp relay information option

Now we need to configure the actual interface via the following commands:

interface fastethernet0/1
ip address 10.10.20.1 255.255.255.0
ip helper-address 10.10.10.5
no shut

We now have a working DHCP relay pointing to our Windows DHCP Server for clients that connect to interface FastEthernet0/1. There are tons more ways to utilize and configure this functionality. For example you could assign “ip helper” to sub-interfaces for vlans. We will dive into the more advanced functionality in later articles, but for now this gives you the basic principal of the commands for configuring DHCP relay.

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