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Embed code for: 21.3 Cisco Device as DHCP Relay Agent
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21.3 Cisco Device as DHCP Relay Agent
Cisco Device as DHCP Relay Agent Section 21: DHCP and Cisco IOS Software Because DHCP clients do not have information about the network to which they are attached, they use UDP broadcasts to send their initial DHCP Discover message. Routers do not normally forward UDP broadcasts because they are configured to not forward broadcast traffic. A primary goal of router configuration is to control unnecessary proliferation of broadcast packets. This means that a DHCP client may be unable to obtain its IP address or other configuration parameters from a DHCP server if the DHCP server is on a different subnet from the DHCP client. It is common, and often desirable, for DHCP servers and clients to reside on different subnets. Instead of using a different DHCP server for each subnet, you might want to use a centralized DHCP solution. A centralized DHCP server enables you to manage IP address assignment in one place for an entire organization. This solution is less time consuming, and it reduces the chance of human error. You can implement a centralized DHCP server solution in which the DHCP clients and the DHCP server reside on different subnets by configuring the DHCP relay agent feature in Cisco IOS software. In the figure, the DHCP client on network 10.1.1.0 will be able to use the DHCP server at 172.23.4.2 because the DHCP relay agent is configured on the router. In this scenario, the DHCP process is as follows: 1. The DHCP client transmits a broadcast DHCP Discover message. 2. When the router receives the broadcast DHCP Discover message, it transmits it to the DHCP server, after it stores one of its own IP addresses in the gateway IP address (GIADDR field) of the DHCP packet. The address that the router stores in the GIADDR field is the address of the interface on which the Discover message was received. 3. The DHCP server uses the GIADDR to determine the subnet of the IP address that it will offer to the client. The DHCP server then reserves an appropriate IP address for the client and sends a DHCP Offer message to the GIADDR address. 4. The DHCP relay agent on the router receives the Offer message and retransmits it to the DHCP client. 5. If the IP address is acceptable, the DHCP client sends a DHCP Request message indicating its intent to accept the parameters in the DHCP offer. 6. The router forwards the DHCP Request message to the DHCP server. 7. When the DHCP server receives the DHCP Request message, it marks the IP address as "in use" in its database and sends a DHCP Acknowledgment (ACK) message. The message includes confirmation for all requested parameters. 8. The router forwards the DHCP ACK message to the DHCP client. At this point, the IP configuration process is complete. To enable the DHCP relay agent on a Cisco device, use the ip helper-address command to configure a helper address on the router interface that is connected to the client. The helper address should specify the IP address of the DHCP server, as shown in the figure: Up Next: Configuring a Cisco Router as a DHCP Server