How to Configure DHCP Relay Agent on Cisco Routers
DHCP clients transmit requests using broadcast messages, which routers do not forward. Consequently, if the DHCP server and clients reside on separate network segments connected by a router, clients will not receive IP configuration information from the DHCP server.
In this scenario, configure the router interface connected to the DHCP server as a DHCP relay agent. Use the following command in interface configuration mode to enable this functionality.
Router(config-if)#ip helper-address [ip-address-of-the-dhcp-server]
The 'ip helper-address' command instructs the interface to forward incoming DHCP messages to the specified DHCP server. Apply this command to all interfaces connected to local subnets containing DHCP clients.
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 link provides a reference for this lab setup. It does not include configuration.
Packet Tracer LAB without configuration
Objective
- Configure PCs in the first four subnets to obtain IP addresses from the DHCP server 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.
Assigning IP configuration
The following commands assign an IP address to a router interface and enable the interface.
| Command | Description (used for) |
| Router>enable | Enter Privilege Exec mode. |
| Router#configure terminal | Enter Global configuration mode. |
| Router(config)#interface [interface name] [interface number] | Enter interface configuration mode. |
| Router(config-if)#ip address [ip address] [subnet mask] | Assign the IP address to the interface. |
| Router(config-if)#clock rate 64000 | 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 | Enable the interface. |
| Router(config-if)#exit | Exit interface configuration mode. |
Router0 (Interface 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)#
Router1 (Interface 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)#
Router2 (Interface 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)#
Configuring RIP routing
Use the following commands to configure RIP routing on a router.
| Command | Description (Used for) |
| Router(config)#router rip | Enable RIP routing, then enter routing configuration mode. |
| Router(config-router)#network [network] | Add the local network to the list of networks that RIP will advertise. |
| Router(config-router)#version 2 | Use RIP version 2. |
| Router(config-router)#no auto-summary | Support classless subnets and disable auto-summary. |
| Router(config-router)#exit | Exit routing configuration mode. |
Router0 (RIP routing configuration)
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)#
Router1 (RIP routing configuration)
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)#
Router2 (RIP routing configuration)
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)#
Assigning a static IP to the DHCP server
A DHCP server requires a static IP configuration. To assign a static IP address, select Server, then Config, and choose the interface connected to Router0's FastEthernet 0/0. In the IP configuration section, select Static and set the IP address.
Because the server connects to the network through Router0's FastEthernet 0/0 interface, use this interface's IP address as the server's default gateway. To configure the default gateway, select the Settings button in the left pane, choose Static in the IP configuration section of the right pane, and enter the default gateway IP address.
Enabling and configuring the DHCP service on the server
To enable the DHCP service on the server, select Server, then Services. In the left pane, select DHCP, and in the right pane, choose the 'On' option. This action enables the DHCP service. The DHCP service uses DHCP pools to allocate IP addresses. A DHCP pool is a collection of IP configurations assigned by the DHCP server to a specific subnet. Create one pool for each subnet; for four subnets, configure four pools.
Entries for the first pool
Entries for the second pool
Entries for the third pool
Entries for the fourth pool
The following image shows the configuration for all the pools.
Configuring DHCP clients
To configure a device as a DHCP client, select the DHCP option in its IP configuration. For example, select PC, then Desktop, then IP Configuration, and choose the DHCP option.
Configure all PCs as DHCP clients by following the steps outlined above.
Configuring DHCP relay agents
Configure the DHCP relay agent only on interfaces connected to local subnets. In this network, four interfaces are directly connected to local subnets, as listed below.
| Router | Interface | Connected to the 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 |
The 'ip helper-address' command designates an interface as a DHCP relay agent. This command requires the DHCP server’s IP address as an argument. Execute this command on interfaces connected to local subnets.
Router1
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)#
Router2
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)#
Verifying DHCP relay agent configuration
Verify the DHCP relay agent configuration by checking the PCs' IP configurations. To do so, select PC, then Desktop, and then IP Configuration. The PC should receive an IP address from its assigned pool.
Alternatively, use the 'ipconfig' command to view the IP configuration.
Configured practice lab
The following link provides the lab configuration above.
Packet Tracer LAB with configuration
This tutorial is part of the tutorial series "DHCP (Dynamic Host Configuration Protocol) basic concepts, configurations, functions, and options explained". Other parts of this series are the following.
Chapter 1 What DHCP is and Types of DHCP Explained
Chapter 2 How DHCP works explained with examples
Chapter 3 DHCP Configuration Parameters and Settings Explained
Chapter 4 How to Configure DHCP Server on Cisco Switches
Chapter 5 Configure DHCP Server for multiple VLANs on the Switch
Chapter 6 How to Configure DHCP Server on Cisco Routers
Chapter 7 How DHCP Relay Agents work Explained
Chapter 8 How to Configure DHCP Relay Agent on Cisco Routers
Chapter 9 How DHCP Snooping works Explained
Chapter 10 Configure DHCP Snooping on Cisco Switches
Conclusion
The 'ip helper-address' command enables the forwarding of DHCP requests across routers, facilitating the deployment of DHCP servers and clients throughout the network. This tutorial demonstrated how to use this command to configure DHCP relay agents when DHCP clients and servers are located on different subnets separated by routers.
By ComputerNetworkingNotes Updated on 2026-02-20