Which command is used on a Cisco router to configure a DHCP relay (helper) address on an interface?
Correct Answer: A
Detailed Explanation: In many networks, DHCP (Dynamic Host Configuration Protocol) servers are centralized and located on a different subnet than the DHCP clients. Since DHCP clients broadcast their DHCP Discover messages to locate a DHCP server, these broadcasts normally do not traverse routers. To overcome this limitation, routers can be configured as DHCP relay agents. The role of the DHCP relay agent is to forward DHCP broadcast messages from the client’s subnet to the DHCP server on another subnet. This is achieved using the "ip helper-address" command. How the DHCP Relay (Helper) Address Works Forwarding Broadcasts: When a DHCP client starts up, it broadcasts a DHCP Discover message to locate a DHCP server. Since routers do not forward broadcasts by default, the DHCP relay agent intercepts this broadcast on the router’s interface that is connected to the client’s subnet. Configuring the Helper Address: To enable this function, the router must be explicitly configured with a DHCP helper address on the interface facing the DHCP clients. The syntax is: Router(config-if)# ip helper-address [DHCP_Server_IP] Here, the [DHCP_Server_IP] is the IP address of the DHCP server located on a different subnet. Once configured, any DHCP broadcast received on that interface is converted into a unicast packet and forwarded to the specified DHCP server. Conversion of Broadcast to Unicast: The router acts as an intermediary. It receives the broadcast from the DHCP client, encapsulates it in a unicast packet, and sends it to the DHCP server. The DHCP server then processes the request and sends a DHCP Offer back to the relay agent, which in turn broadcasts the offer to the client. Multiple Helper Addresses: In some networks, it may be necessary to forward DHCP broadcasts to more than one server for redundancy. Cisco routers support multiple helper addresses on a single interface. This ensures that if one DHCP server is unavailable, another can respond. Additional UDP Ports: The ip helper-address command does more than simply forward DHCP messages (which use UDP port 67 and 68). By default, the command also forwards several other UDP broadcasts (such as TFTP, DNS, and NetBIOS) to the helper address. This behavior can be modified if necessary, but in many cases, it provides additional convenience for centralized services. Troubleshooting: If DHCP clients are not receiving IP addresses, one of the first troubleshooting steps is to verify that the router interface connected to the client’s subnet has the correct helper address configured. The command "show running-config" can be used to confirm the helper address. Additionally, packet capture tools can be used to ensure that DHCP Discover packets are being forwarded correctly. Conclusion The "ip helper-address" command (Option A) is the correct and standard method for configuring a DHCP relay agent on a Cisco router interface. This command ensures that DHCP broadcast traffic is properly forwarded to a centralized DHCP server located on another subnet, thereby enabling dynamic IP address allocation across multiple subnets. Mastery of this command is essential for CCNA candidates and network professionals responsible for designing and troubleshooting DHCP services in enterprise networks.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.