DHCP Relay Agent Configuration

Now that we've answered the question of what a DHCP relay agent is, let's look at how to configure one on a Cisco router.

PC1 Before the Relay

Before configuring the relay, let's look at PC1:

C:\> ipconfig

FastEthernet0 Connection:(default port)

   Connection-specific DNS Suffix..: 
   Link-local IPv6 Address.........: FE80::2E0:F9FF:FE23:D3B5
   IPv6 Address....................: ::
   IPv4 Address....................: 0.0.0.0
   Subnet Mask.....................: 0.0.0.0
   Default Gateway.................: ::
                                     0.0.0.0

No IPv4 address, no subnet mask, no default gateway. PC1 sent a DHCP DISCOVER, but the broadcast did not cross the router and never reached the server.

Figure 1 – DHCP Relay Configuration on a Cisco Router

The configuration is straightforward.

Configuring a DHCP Relay Agent on Cisco Devices

1. Access the router's configuration mode:

   Start CLI

R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.

2. Navigate to the interface connected to the local subnet (where the client resides):

   Start CLI

R1(config)# int G0/0

3. Set the IP address of the DHCP server as the helper address:

   Start CLI

R1(config-if)# ip helper-address 192.168.2.2
R1(config-if)# end

The ip helper-address command enables the router to forward DHCP requests received on the interface to the server at 192.168.2.2.

After setting up your DHCP relay agent, it's essential to verify that it's functioning as expected.
Let's explore how you can confirm your configuration and troubleshoot any issues.