Cisco IP helper address explained with example lab and check how we can use IP helper address command in our network
The Cisco IP helper address informs the router to forward the DHCP packets to the defined destination. Usually, routers do not forward broadcast traffic so by default they drop any broadcast packet that they receive.
DHCP service uses broadcast traffic to communicate with other devices in the network so if we want routers to forward the DHCP traffic then we have to define the destination address for the broadcast traffic using the IP helper address command.
Although the DHCP server should be on the same network so we don’t have to pass the DHCP traffic through the routers, due to some specific network design requirements, if we have to use the DHCP server through the routers then we can use IP helper-address command to pass the traffic from one network to other.
Using this feature increases the processor cycle of the router as it has to handle all DHCP traffic and if there are a large number of end devices that generate DHCP packets regularly then it will affect the performance of the router.
We have to use the IP helper-address command on the interface of the router that is receiving the broadcast traffic.
In this example, PCs are configured to get IPs through the DHCP server so they will be generating the DHCP broadcast packets, and router 0 will receive those packets on a Gigabit Ethernet 0/0/1 port.
We will use this command on the interface configuration mode and will define the forwarding address which is the IP address of the gigabit 0/0 interface of router 2.
In the below animation, we can see that router 0 is dropping the DHCP traffic because we have not yet configured the IP helper address on the router’s interface.
We have configured that IP helper address and the router is now forwarding the DHCP traffic to router 2.
We have configured DHCP service on router 2 so routing 2 can assign the IP addresses to the PCs configured to obtain IP addresses from the DHCP server.
Router 2 has replied to the DHCP IP request and successfully assigned the IP addresses to both of the PCs situated on the other network.
We can see in the image below that IPs are assigned to the PCs via DHCP service.
You can download the lab to check the IP helper address configuration. Two labs are available to download below.
Lab without the IP helper address configuration
Lab with the IP helper address configuration
