How DHCP Relay agent works?

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How Relay Agents Work

The following figure shows how Client C on Subnet 2 obtains a DHCP address lease from DHCP Server 1 on Subnet 1.
Using a Relay Agent
Using a Relay Agent
  1. DHCP Client C broadcasts a DHCPDiscover message on Subnet 2 as a UDP datagram over well-known UDP port 67, which is the port reserved and shared for BOOTP and DHCP server communication.
  2. The relay agent, in this case a DHCP relay-enabled router, examines the Gateway IP Address field (also known as the giaddr field) in the DHCP message header. If the field has an IP address of 0.0.0.0, the agent fills the Gateway IP Address field with the IP address assigned to the interface on which the DHCPDiscover was received, and forwards the DHCPDiscover message as a unicast message to the DHCP server on Subnet 1.
  3. When DHCP Server 1 receives the DHCPDiscover message, it examines the Gateway IP Address field to determine if the packet was relayed. The DHCP server then determines whether it can supply an IP address lease to clients on the subnet indicated by the address in the Gateway IP Address field.

    For example, if the Gateway IP Address field has an IP address of 192.168.45.2, the DHCP server checks its DHCP scopes for a scope range that includes 192.168.45.2 (the gateway IP address in the packet). To find a match, the DHCP server determines whether the IP address 192.168.45.2 matches the network ID of each scope. It determines the network ID of each scope by ANDing any IP address in the scope with its corresponding subnet mask. In this case, the DHCP server checks to see which scope includes addresses for Subnet 2. If a scope exists that matches this criterion, the DHCP server selects an available address from the matched scope to use in an IP address lease offer response to the client.
  4. DHCP Server 1 sends a DHCPOffer message directly to the relay agent identified in the Gateway IP Address field.
  5. The relay agent relays the DHCPOffer message to the DHCP client.

    Depending on the value of the Broadcast flag in the DHCPOffer message (which was copied from the DHCPDiscover message), the DHCP relay agent either unicasts or broadcasts the DHCPOffer message.
  6. By using a similar process, a DHCPRequest message is relayed from client to server, and a DHCPAck message is relayed from server to client.                                                                                    




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