DHCP Relay Agent

Overview

In principle, the DHCP server and DHCP clients must be connected on the same network. The DHCP Relay Agent is the function to avoid this restriction.

This section explains how the DHCP Relay Agent works and the configuration commands on a Cisco router (Cisco IOS).

Related article

Please also refer to the following article about DHCP.

N-Study

DHCP : Automatically Assign IP Address | N-Study

https://www.n-study.com/en/tcp-ip/dhcp-overview/

DHCP is the protocol used to automatically distribute IP addresses and other TCP/IP configurations.DHCP is a very commonly used protocol. It is important to understand how it works.

What is DHCP Relay Agent?

In principle, the DHCP server and DHCP clients must be connected to the same network. This means that DHCP servers must be deployed in each network. It is very time-consuming to deploy many DHCP servers and manage each server.

Therefore, the DHCP Relay Agent is used so that a single DHCP server can distribute TCP/IP configuration information to DHCP clients on multiple networks. The DHCP Relay Agent forwards broadcast DHCP messages from DHCP clients as unicasts to the DHCP server. In most cases, routers are configured as DHCP relay agents.

How the DHCP Relay Agent works

Configure DHCP pools for multiple networks on DHCP server

In order to distribute configuration information to DHCP clients on multiple networks with single DHCP server, DHCP pools for multiple networks should be configured in advance.

For example, to distribute TCP/IP configuration information to DHCP clients on three networks, 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24, create three DHCP pools as shown in the following figure.

In considering how the DHCP Relay Agent works, let’s look at the IP address assignment to DHCP client 1 on 192.168.1.0/24 in the above figure.

DHCP DISCOVER Forwarding

When DHCP Client 1 connects to the network, it broadcasts DHCP DISCOVER message to find a DHCP server. The broadcast is not forwarded to other networks.

The router with the DHCP relay agent enabled receives the DHCP DISCOVER message and translates the destination IP address to the unicast IP address of the DHCP server and forwards it. At that time, the IP address of the relay agent, 192.168.1.1, is specified as gipaddr in the DHCP message.

The DHCP server receives a unicast DHCP DISCOVER message and must determine which network the DHCP client is on. So, it checks the gipaddr of the DHCP relay agent’s IP address in the DHCP message.The DHCP Relay Agent should be on the same network as the client. The DHCP server distributes the IP address of DHCP Pool 1, which is the same network as the IP address of the DHCP Relay Agent.

DHCP OFFER Forwarding

The DHCP server forwards DHCP OFFER, which provide configuration information to the client, to the relay agent. The relay agent then forwards it to the entire 192.168.1.0/24 address with the destination as a broadcast address. DHCP client 1 can then receive the DHCP OFFER.

DHCP REQUEST Forwarding

DHCP client 1 broadcasts DHCP REQUEST to request the TCP/IP configuration information proposed in the OFFER. The DHCP Relay Agent will then forward the information to the DHCP server via unicast.

DHCP ACK Forwarding

Forwards ACK message from DHCP server to relay agent authorizing use of TCP/IP configuration information. The relay agent forwards the DHCP ACK message to the client via broadcast. DHCP client 1 now completes the TCP/IP configuration, including the IP address.

DHCP Relay Agent Configuration and Verification Commands (Cisco IOS)

DHCP Relay Agent Configuration Commands

You can configure a Cisco router as a DHCP relay agent. Enter the configuration command on the interface that will receive broadcast DHCP messages. That is, the interface to which the DHCP client is connected.

The configuration command is very simple: configure the ip helper-address command on every interface where DHCP clients are present.

DHCP Relay Agent Verification Commands

The following table summarizes the main commands used to verify that the DHCP Relay Agent has been configured correctly.

show ip interface

Verify that the IP address of the DHCP server to which DHCP messages are forwarded is configured correctly with the show ip interface command.

R1#show ip interface Ethernet 0/0
Ethernet0/0 is up, line protocol is up
  Internet address is 192.168.1.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is 192.168.4.100
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  access list is not set
  Proxy ARP is enabled
-- omitted --

Summary

Related article

The following article is a detailed configuration example of the DHCP Relay Agent on a Cisco router. The packet capture results are also included so that you can see in detail how the DHCP Relay Agent works.

N-Study

DHCP Relay Agent Configuration Example [Cisco] | N-Study

https://www.n-study.com/en/tcp-ip/cisco-dhcp-relay-agent-configuration-example/

Enable DHCP Relay Agent on Cisco routers (Cisco IOS). This allows a single DHCP server to support DHCP clients on multiple networks.

The following article summarizes DHCP server placement.

N-Study

Summary of DHCP Server Placement | N-Study

https://www.n-study.com/en/tcp-ip/dhcp-server-placement/

In principle, the DHCP server and client must be on the same network; this section summarizes how to place the DHCP server.