DHCP Relay Agent

1. What Is a DHCP Relay Agent?

Dynamic Host Configuration Protocol (DHCP) simplifies IP address assignment.

But what happens when a DHCP server and a client reside on different subnets?

Normally, broadcast messages like DHCP requests do not cross routers.

What is a DHCP Relay Agent – Diagram showing DHCP request blocked by router when client and server are on different subnets

This creates a problem for DHCP clients in remote networks because they cannot communicate with the DHCP server to obtain an IP address.

If you’re wondering what is a DHCP relay agent, it’s the component that solves exactly this problem. A relay agent intercepts DHCP requests from clients and forwards them to a DHCP server located in another subnet.

Network diagram showing how a router forwards a DHCP request from a PC in one subnet to a DHCP server in another subnet.

As you can see here, R1 is the DHCP Relay that sends the DHCP message to the DHCP Server located in another subnet.

Let’s take a closer look at why and how a DHCP Relay Agent can solve this challenge.

Why We Need a DHCP Relay Agent

Consider this scenario:

Diagram showing that a DHCP Discover broadcast from PC1 does not reach the DHCP server because routers block broadcasts across subnets.
  • A DHCP Client (PC1) in subnet 192.168.1.0/24 sends a DHCP DISCOVER broadcast message to obtain an IP address.
  • The router (R1) receives the request but because broadcast traffic cannot traverse routers, the message does not reach the DHCP server in subnet 192.168.2.0/24.

Without a DHCP Relay Agent, PC1 would remain unconfigured and unable to connect to the network. By configuring R1 as a relay agent, the DHCP request is forwarded to the server and the server’s response is relayed back to the client.

Overview: What Is a DHCP Relay Agent and How It Works

A DHCP Relay Agent is a service configured on layer 3 devices such as routers or Layer 3 switches. It listens for DHCP requests and forwards them as unicast messages to the DHCP server.

Diagram showing how a DHCP Relay Agent forwards requests between clients and DHCP servers on different subnets.

Once the DHCP server responds, the relay agent ensures that the response is forwarded back to the client.

2. DHCP Relay Configuration

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

Topology showing DHCP Relay configuration with router forwarding requests between clients and DHCP server

The configuration is straightforward.

Configuring a DHCP Relay Agent on Cisco Devices

  1. Access the router’s configuration mode:
R1> enable  
R1# configure terminal

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

R1(config)# interface GigabitEthernet0/0

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

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

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

3. Verify DHCP Relay

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.

Verifying the DHCP Relay Agent Configuration

To verify that your configuration is working correctly, use the show ip dhcp relay command.

R1# show ip dhcp relay
Interface          Helper address              VRF  
---------------------------------------------------  
GigabitEthernet0/0 192.168.2.2                 global

Here we can view that the relay agent configured on GigabitEthernet0/0 will forward DHCP requests to the server at 192.168.2.2.

Monitor DHCP Events in Real-Time

You can monitor DHCP activity as the relay agent facilitates communication between the DHCP Client and DHCP Server. Let’s walk through the process using the debug ip dhcp events command:

R1# debug ip dhcp events  

*Mar  1 00:02:03.123: DHCPD: DHCPDISCOVER received from client 0100.1c42.4c39.91 on interface GigabitEthernet0/0 

*Mar  1 00:02:03.124: DHCPD: Forwarding DHCPDISCOVER to helper address 192.168.2.2 
 
*Mar  1 00:02:04.567: DHCPD: DHCPOFFER received from server 192.168.2.2  

*Mar  1 00:02:04.568: DHCPD: Forwarding DHCPOFFER to client 0100.1c42.4c39.91 on interface GigabitEthernet0/0  

*Mar  1 00:02:05.890: DHCPD: DHCPREQUEST received from client 0100.1c42.4c39.91 on interface GigabitEthernet0/0  

*Mar  1 00:02:05.891: DHCPD: Forwarding DHCPREQUEST to helper address 192.168.2.2 

*Mar  1 00:02:06.012: DHCPD: DHCPACK received from server 192.168.2.2  

*Mar  1 00:02:06.013: DHCPD: Forwarding DHCPACK to client£ 0100.1c42.4c39.91 on interface GigabitEthernet0/0

This detailed output confirms that the entire DORA process (DISCOVER, OFFER, REQUEST, ACK) has been completed successfully.

DHCP Relay Process in Action

Let’s walk through how a DHCP relay agent facilitates IP address allocation.

Let’s break this down step by step:

DHCP DISCOVER: The Client Searches for a DHCP Server

When the client sends a DHCPDISCOVER broadcast message, the relay agent intercepts it and forwards it as a unicast to the DHCP server (192.168.2.2).

Router forwarding a DHCP DISCOVER broadcast from client to a remote DHCP server through DHCP Relay

Since broadcast messages cannot cross routers, the relay agent ensures the request reaches the DHCP server.

DHCP OFFER: The Server Makes an Offer

The DHCP server responds with a DHCPOFFER, providing the client with an available IP address and configuration details. The relay agent forwards this response to the client.

Alt Text : DHCP Offer packet sent back to the client via the relay agent

This is where the client gets its first look at the offered network settings.

DHCP REQUEST: The Client Accepts the Offer

The client replies with a DHCPREQUEST message to indicate it accepts the offer. The relay agent forwards this message to the DHCP server.

Diagram showing the DHCP Request sent from client to server via relay agent

This step confirms that the client is ready to use the assigned IP address, along with the subnet mask, default gateway, DNS server and lease duration.

DHCP ACK: The Server Confirms the Allocation

Finally, the server sends a DHCPACK message to confirm the IP address allocation and configuration. The relay agent ensures this message reaches the client.

Diagram showing the DHCP ACK response sent from server to client via relay agent

The client can now configure its network settings and begin communication on the network.

4. Conclusion

So, what is a DHCP relay agent really about? It’s a crucial component that makes dynamic IP assignment possible when clients and DHCP servers are located on different subnets.

Instead of allowing DHCP broadcast messages to fail at the router boundary, the relay agent captures and forwards them as unicast packets to the correct DHCP server, then brings the response back to the client.

Here’s what you’ve learned:

  • How to configure it using the ip helper-address command on Cisco routers
  • What happens during the full DORA exchange (Discover, Offer, Request, Acknowledgment)
  • How to verify and debug the setup with real Cisco CLI commands

This mechanism may seem simple, but it’s a fundamental piece of scalable network design and it’s something you need to master for the CCNA exam and real-world implementation.