How to Configure DHCP Relay Agent on Cisco Router in GNS3

In our previous article, we looked at how to configure DHCP Server on Cisco Routers. In this article, we will examine how automatic IP addresses are assigned to clients from DHCP Server on a different network.

How to Configure DHCP Relay Agent on Cisco Router in GNS3

How to Configure a DHCP Relay Agent on a Router

If the DHCP server configured on a network becomes unresponsive for any reason, a DHCP server can be configured as a backup on a different network.

Thus, when the DHCP server on LAN1 fails to respond to clients’ IP address request messages, the DHCP server on LAN2 will automatically respond.

In this article, we will configure the Relay Agent service by creating a small network topology using VMware Workstation and GNS3.

Create two virtual machines with VMware and configure VMnet settings. In the GNS3 program, add Cloud Node to these virtual machines.

How to Enable DHCP Relay Agent

Follow the steps below to configure the DHCP Relay Agent service on Cisco Routers.

   Step 1

Open the GNS3 program installed on your computer and create a new project.

Creating a New Project in GNS3

   Step 2

Create a network topology for the DHCP Relay Agent as in the following image.

Two Switches Connected to Router in GNS3

   Step 3

Configure the FastEthernet 0/0 and FastEthernet 0/1 interfaces on the Cisco Router.

R1# conf t
R1(config)# interface fastethernet 0/0
R1(config-if)# ip address
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface fastethernet 0/1
R1(config-if)# ip address
R1(config-if)# no shutdown
R1(config-if)# end

R1(config-if)# ip address

   Step 4

Create a DHCP pool as an example on Router R1 and enter R1DHCP as the domain-name. First, we will configure the DHCP server running on the Cisco Router to assign an automatic IP address to the Windows 8.1 virtual machine.

We will then install a 3rd party DHCP server application on the Windows 10 virtual machine and configure the necessary settings for the DHCP Relay Agent on the Router.


   Step 5

Enable the automatic IP address option in the TCP/IP settings of the Windows 8.1 virtual machine.

Configure Windows Virtual Machine TCP/IP Settings

   Step 6

Close the TCP/IPv4 preferences window by clicking the OK button.

Windows Automatic IP Address Retrieval

   Step 7

In the Windows 8.1 VM, click the Ethernet Details button and check the IP address settings. Verify that an automatic IP address obtain from the DHCP server configured on the Cisco Router as shown in the image below.

Network Connection Details

   Step 8

Go to the Windows 10 virtual machine and test the network connection by pinging the Win8.1 VM.

Pinging through CMD

   Step 9

Download the 3rd Party DHCP Server from the button at the bottom of the text. Run dhcpwiz.exe file in the folder and click Next to continue.


   Step 10

Pay attention to the setting here! The IP address will be the IP address of the DHCP server.

Network Card Settings

   Step 11

You can examine the protocol and services supported by the program from the image below. Here, continue with Next without selecting any service.

Supported Protocols

   Step 12

Because the Switch to which the Windows 8.1 VM is connected to the FastEthernet 0/0 interface of the Cisco Router, any device to be connected to this Switch will receive an IP address from the IP address block.

However, if the DHCP configuration configured for FastEthernet 0/0 on the router cannot serve for any reason, at this point you must enable a DHCP Relay service that provides services in the FastEthernet 0/1 area to maintain network connectivity.

Therefore, you must install the DHCP Server program on a computer in the FastEthernet 0/1 domain to create an IP pool for block

After configuring the DHCP Pool as in the image below, click the DHCP Options button.

DHCP Settings for the Interface

   Step 13

In the DHCP Options window, type DHCPRELAY in the domain name field and click OK.


   Step 14

In the DHCP Server Advanced window, configure the Subnet Mask and Default Gateway options as follows.

Advanced Configuration

   Step 15

After configuring the DHCP Server on the Windows 10 VM, click Next to continue to the next step.

IP Pool Configuration

   Step 16

It is important to note that the information shown in the summary is required to save to a file. Select the Overwrite existing file option, and then click the Write INI file button to save the settings.

Writing Changes to the INI File

   Step 17

To start DHCP service, the following window will pop up and click the Admin button.

Run DHCP Server Now

   Step 18

From the Service section, click Install.

DHCP Service Setup

   Step 19

The DHCP service is currently down. Click the Start button to start the DHCP service.

Starting the DHCP Service

   Step 20

In the Status section, you can see that the DHCP Server is running smoothly!

DHCP Service Status

   Step 21

Now open the Cisco Router CLI command prompt and type the 3rd Party DHCP server IP address using the ip helper-address command on the FastEthernet 0/0 interface.

Use the no ip dhcp pool LAN command to disable the DHCP service running on the router’s Fa0/0 interface, assuming it has encountered an error.

R1# conf t
R1(config)# interface fastethernet 0/0
R1(config-if)# ip helper-address
R1(config-if)# exit
R1(config)# no ip dhcp pool LAN

ip helper-address

   Step 22

To allocate another IP address from the DHCP server on the Windows 8.1 VM, open the CMD prompt and execute the ipconfig /release command.

ipconfig /release

   Step 23

Now let’s get a new IP address by applying the ipconfig /renew command.

ipconfig /renew

   Step 24

The virtual machine has successfully obtained IP address information from the DHCP Server with the IP address running on the Windows 10 VM on the network.

Please pay attention to the domain name in the image below.

Network Connection Details

How to Use Relay Agent to Obtain an IP Address from a Different Network ⇒ Video

You can watch the video below to use the DHCP Relay Agent and also subscribe to our YouTube channel to support us!

   Final Word

In this article, we have examined how to obtain an automatic IP address from a DHCP server on a different network. You can consider the Relay Agent service as an auxiliary DHCP server.

If we had not configured Relay Agent on the Router, the Windows 8.1 machine would not be able to obtain the IP address! Thanks for following us!

   Related Articles

Cisco VLAN Configuration
♦ GNS3 VM Installation
♦ GNS3 Port Security Configuration
Cisco OSPF Configuration
Cisco RIP Configuration

   Download Software

You can download the 3rd Party DHCP Server program from the button below.

Download Software

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