DHCP clients use broadcast messages to send DHCP requests. Routers do not forward broadcast messages. If the DHCP server and clients are in different network segments and connected through a router, the clients will not receive IP configurations from the DHCP server.
In such a situation, you need to configure the router's interface connected to the DHCP server as a DHCP relay agent. To configure a router's interface as a DHCP relay agent, use the following command in interface configuration mode.
Router(config-if)#ip helper-address [ip-address-of-the-dhcp-server]
The 'ip helper-address' command tells the interface to forward the incoming DHCP message to the configured DHCP server. Use this command on all interfaces that are connected to the local subnets containing DHCP clients.
For this tutorial, I assume that you know how a DHCP relay agent functions. To learn what a DHCP relay agent is and how it works, please check the previous parts of this tutorial.
This tutorial is the eighth part of the tutorial 'DHCP (Dynamic Host Configuration Protocol) basic concepts, configurations, functions, and options Explained'. Other parts of this tutorial are the following.
What DHCP is and Types of DHCP Explained
How DHCP works explained with examples
DHCP Configuration Parameters and Settings Explained
How to Configure DHCP Server on Cisco Switches
Configure DHCP Server for multiple VLANs on the Switch
How to Configure DHCP Server on Cisco Routers
How DHCP Relay Agents work Explained
How DHCP Snooping works Explained
Configure DHCP Snooping on Cisco Switches
Packet tracer practice lab setup
- Create a network that contains five subnets: 10.0.0.0/8, 20.0.0.0/8, 30.0.0.0/8, 40.0.0.0/8, and 50.0.0.0/8.
- Add some hosts in each network except the fifth subnet. In the fifth subnet add a DHCP Server.
- To connect subnets, use three routers: Router0, Router1, and Router2.
- Connect subnets 10.0.0.0/8 and 20.0.0.0/8 to Router1's FastEthernet 0/0 and FastEthernet0/1 interfaces, respectively.
- Connect subnets 30.0.0.0/8 and 40.0.0.0/8 to Router2's FastEthernet 0/0 and FastEthernet0/1 interfaces, respectively.
- Connect the serial 0/0/0 interface of Router1 to the serial 0/0/0 interface of Router0. Use the subnet 192.168.1.0/252 for the IP configuration on this connection.
- Connect the serial 0/0/1 interface of Router2 to the serial 0/0/1 interface of Router0. Use the subnet 192.168.4.0/252 for the IP configuration on this connection.
- Configure RIP routing between routers and test connectivity between subnets.
The following image shows this network.
Objective
- Configure the hosts of the first four subnets to obtain IP addresses from the DHCP server that is available in the fifth subnet.
- On the DHCP server, create four DHCP pools and assign one pool to each subnet.
- Configure routers to act as DHCP relay agents.
Pre-created practice lab
Instead of replicating this lab, if you want to download and use the practice lab used in this tutorial, you can download it from the following link.
Packet Tracer LAB without configuration
Let's configure this lab and complete all the above-listed objectives step by step.
Assigning IP configuration
The following table lists all necessary commands to assign IP configuration on a router's interface.
| Command | Description (used for) |
| Router>enable | To start the command prompt and to change the command mode to privileged-exec mode. |
| Router#configure terminal | To navigate to global configuration mode. |
| Router(config)#interface [interface name] [interface number] | To navigate to interface mode |
| Router(config-if)#ip address [ip address] [subnet mask] | To set the IP address on the interface |
| Router(config-if)#clock rate 64000 | To set the clock rate to 64000. (Use only if the interface is serial DCE) |
| Router(config-if)#bandwidth 64 | To set the bandwidth to 64 (Use only if the interface is serial DCE) |
| Router(config-if)#no shutdown | To enable the interface |
| Router(config-if)#exit | To exit interface mode |
If you want to learn more about the above commands, you can check the following tutorial.
RIP Protocol configuration Guide with Examples
This tutorial explains how to enable a Cisco router's interface and how to configure RIP routing on Cisco routers in detail.
Access the command prompt of each router and configure its interfaces as shown below.
Router0 IP configuration
Router>enable Router#configure terminal Router(config)#interface fastethernet 0/0 Router(config-if)#ip address 50.0.0.1 255.0.0.0 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface serial 0/0/0 Router(config-if)#ip address 192.168.1.2 255.255.255.252 Router(config-if)#clock rate 64000 Router(config-if)#bandwidth 64 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface serial 0/0/1 Router(config-if)#ip address 192.168.1.6 255.255.255.252 Router(config-if)#clock rate 64000 Router(config-if)#bandwidth 64 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#
The following image shows the above configuration on the packet tracer.
Router1 IP configuration
Router>enable Router#configure terminal Router(config)#interface fastethernet 0/0 Router(config-if)#ip address 10.0.0.1 255.0.0.0 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface fastethernet 0/1 Router(config-if)#ip address 20.0.0.1 255.0.0.0 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface serial 0/0/0 Router(config-if)#ip address 192.168.1.1 255.255.255.252 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#
The following image shows the above configuration on the packet tracer.
Router2 IP configuration
Router>enable Router#configure terminal Router(config)#interface fastethernet 0/0 Router(config-if)#ip address 30.0.0.1 255.0.0.0 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface fastethernet 0/1 Router(config-if)#ip address 40.0.0.1 255.0.0.0 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#interface serial 0/0/1 Router(config-if)#ip address 192.168.1.5 255.255.255.252 Router(config-if)#no shutdown Router(config-if)#exit Router(config)#
The following image shows the above configuration on the packet tracer.
Configuring RIP routing
The following table lists all necessary commands to configure RIP routing on a router.
| Command | Description (Used for) |
| Router>enable | To enter privileged-exec mode |
| Router#configure terminal | To enter global configuration mode |
| Router(config)#router rip | To enable RIP routing and enter routing configuration mode |
| Router(config-router)#network [network] | To add the locally available network to the list of networks that the RIP will advertise. |
| Router(config-router)#version 2 | To use the RIP version 2. |
| Router(config-router)#no auto-summary | To support the classless subnets or to disable the auto-summary feature. |
| Router(config-router)#exit | To exit from routing configuration mode. |
Access the CLI prompt of each router and configure RIP routing as shown below.
Router0
Router>enable Router#configure terminal Router(config)#router rip Router(config-router)#network 50.0.0.0 Router(config-router)#network 192.168.1.0 Router(config-router)#network 192.168.4.0 Router(config-router)#version 2 Router(config-router)#no auto-summary Router(config-router)#exit Router(config)#
The following image shows the above configuration on the packet tracer.
Router1
Router>enable Router#configure terminal Router(config)#router rip Router(config-router)#network 10.0.0.0 Router(config-router)#network 20.0.0.0 Router(config-router)#network 192.168.1.0 Router(config-router)#version 2 Router(config-router)#no auto-summary Router(config-router)#exit Router(config)#
The following image shows the above configuration on the packet tracer.
Router2
Router>enable Router#configure terminal Router(config)#router rip Router(config-router)#network 30.0.0.0 Router(config-router)#network 40.0.0.0 Router(config-router)#network 192.168.1.4 Router(config-router)#version 2 Router(config-router)#no auto-summary Router(config-router)#exit Router(config)#
The following image shows the above configuration on packet tracer.
Assigning static IP to DHCP server
Before we configure the DHCP service on the server, we have to assign a static IP address to the server.
To assign a static IP address to the server, click Server and click Config and click the interface that is connected to Router0's FastEthernet 0/0. In the IP configuration section, select the Static option, and set the IP address.
The following image shows this process.
Since the server is connected to the network through the Router0's FastEthernet 0/0 interface, we will set the IP address of Router0's FastEthernet 0/0 interface as the default gateway IP address on the server.
To set default gateway IP, click the Settings button in the left pane and choose the Static option in the IP configuration section of the right pane and set the IP address of the default gateway.
The following image shows this process.
Enabling and Configuring DHCP Service on the Server
To enable DHCP service on the Server, click Server and click Services. In the left pane click DHCP and in the right pane select the 'On' option. This will enable the DHCP service.
DHCP service uses DHCP pools to provide IP addresses. A DHCP pool is a group of IP configurations that the DHCP server assigns to a specific subnet. We have to create pools equal to subnets. In our network, since we have four subnets, we have to create four pools.
To create and add a pool, fill entries and click the Add button. Make sure that you add the correct default gateway IP in each pool. DHCP service uses the default gateway IP to identify the subnet.
The following image shows the entries for the first pool.
The following image shows the entries for the second pool.
The following image shows the entries for the third pool.
The following image shows the entries for the fourth pool.
The following image shows all the pools configured.
Configuring DHCP clients
To configure the device as a DHCP client, we need to select the DHCP option in its IP configuration. To do this, click the device and click Desktop and click the IP Configuration and select the DHCP option.
The following image shows how to set the DHCP option in the IP configuration section.
Set all PCs as DHCP clients by following the steps described above.
Configuring DHCP relay agents
We configure a DHCP relay agent only on the interface that is directly connected to a local subnet. In this network, four interfaces are directly connected to the local subnets. These interfaces are the following.
| Router | Interface | Connected local subnet |
| Router1 | Fa0/0 | 10.0.0.0/8 |
| Router1 | Fa0/1 | 20.0.0.0/8 |
| Router2 | Fa0/0 | 30.0.0.0/8 |
| Router2 | Fa0/1 | 40.0.0.0/8 |
To configure the above-listed interfaces as DHCP relay agents, use the 'ip helper-address' command as shown below.
Router1
Router>enable Router#configure terminal Router(config)#interface fastethernet 0/0 Router(config-if)#ip helper-address 50.0.0.10 Router(config-if)#exit Router(config)#interface fastethernet 0/1 Router(config-if)#ip helper-address 50.0.0.10 Router(config-if)#exit Router(config)#
The following image shows the above configuration.
Router2
Router>enable Router#configure terminal Router(config)#interface fastethernet 0/0 Router(config-if)#ip helper-address 50.0.0.10 Router(config-if)#exit Router(config)#interface fastethernet 0/1 Router(config-if)#ip helper-address 50.0.0.10 Router(config-if)#exit Router(config)#
The following image shows the above configuration.
Verifying DHCP relay
To verify that all DHCP relays are configured properly, check the IP configuration of their connected devices. To check the IP configuration of a client device, click the device and click the Desktop option and click the IP configuration. If the device shows the correct IP configuration, the DHCP relay is configured properly.
The following image shows this process.
If you prefer the command prompt, you can use the 'ipconfig' command to view the received IP configuration. The following image shows how to view IP configuration on PC2 at the command prompt.
Configured practice lab
The following download link provides the configured LAB of this example.
Packet Tracer LAB with configuration
That's all for this part. In the next part of this tutorial, we will understand what the DHCP snooping is and how it works.
