DHCP Relay Agent

A DHCP (Dynamic Host Configuration Protocol) Relay Agent is a network device or software component that helps facilitate the distribution of IP addresses and configuration information to DHCP clients in different network segments or subnets. When a DHCP server is located on a different subnet from the DHCP clients, a relay agent is necessary to forward DHCP requests and responses between the clients and the server.

Fig 1:DHCP Relay Agent

The primary function of a DHCP Relay Agent is to receive DHCP DISCOVER packets from DHCP clients, encapsulate them into DHCP relay messages, and then forward these messages to a DHCP server. When the DHCP server receives the relayed message, it processes it and sends back the DHCP offer, acknowledgment, or other relevant information to the relay agent, which then forwards the response to the DHCP client.

Configuring a DHCP Relay Agent typically involves the following steps:

1. Identify the Relay Agent: 

Determine which router or device in your network will act as the DHCP Relay Agent. This device should be located on the subnet where the DHCP clients are present and should have an interface on both the client's subnet and the subnet where the DHCP server is located.

2. Configure the Relay Agent:

   - Enable the DHCP Relay Agent functionality on the selected device.

   - Specify the IP address of the DHCP server(s) on interface.

   - Optionally, configure additional relay agent settings like timeouts and maximum hop counts.

3. Relay DHCP Requests:

   - When a DHCP client on client's subnet sends a DHCPDISCOVER message, the relay agent intercepts it.

   - The relay agent encapsulates the DHCPDISCOVER message into a DHCP relay message.

   - The relay agent forwards the DHCP relay message to the configured DHCP server(s).

4. Forward DHCP Responses:

   - When the DHCP server on the server's subnet processes the relayed DHCP request, it sends back a            DHCP offer, acknowledgment, or other responses to the relay agent.

   - The relay agent receives the DHCP response and forwards it back to the DHCP client on the client's subnet.

By using a DHCP Relay Agent, you can centralize the management of IP address assignments and configuration settings, making it easier to manage DHCP services in large and segmented networks. DHCP Relay Agents are commonly used in scenarios where multiple subnets or VLANs are in use, and a single DHCP server needs to serve clients in different subnets.

Cisco IOS Configurations of DHCP Relay Agent

enable

configure terminal

interface <interface_type> <interface_number>

ip helper-address <dhcp_server_ip>