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Difficulty: MediumDHCP Server, Client, and Relay Agent Operation and Configuration

Host devices connected to an access switch on VLAN 15 (172.16.15.0/24172.16.15.0/24) are failing to receive automatic IPv4 address assignments. Router R1 serves as the default gateway via its GigabitEthernet0/0.15 subinterface, while the enterprise DHCP server is located at 10.1.1.25010.1.1.250 across the WAN backbone. To enable successful DHCP relay operation for hosts on VLAN 15, on which interface must the `ip helper-address 10.1.1.250` command be configured on router R1?

  1. On subinterface GigabitEthernet0/0.15, because the DHCP relay agent must intercept broadcast DISCOVER requests on the client-facing ingress gateway interface.Answer
  2. B
    On the WAN interface connected towards the DHCP server, because helper commands direct egress traffic out of the router.
  3. C
    On the access switch ports connected to the hosts, because Layer 2 interfaces must rewrite DHCP broadcast frames into unicast frames before forwarding.
  4. D
    On the global configuration mode of router R1 without an interface context, because relay addresses apply router-wide across all active subnets.

Answer

The helper address command must be configured on subinterface GigabitEthernet0/0.15, which is the client-facing ingress interface serving VLAN 15.
The correct answer specifies configuring the command on subinterface GigabitEthernet0/0.15. A Cisco IOS router acts as a DHCP relay agent by intercepting UDP broadcast traffic (ports 67 and 68) on the incoming client-facing interface, inserting its local interface address into the gateway IP address (giaddr) field of the DHCP packet header, and forwarding the request as a unicast packet to the designated DHCP server IP address.

Step-by-Step Solution

1
Identify the ingress interface receiving client broadcasts
GigabitEthernet0/0.15 is the local default gateway interface receiving client DHCP DISCOVER broadcast frames on VLAN 15.
DHCP relay functions by listening for Layer 2 broadcasts on the client-facing gateway interface.
2
Apply the IP helper address configuration
Enter subinterface configuration mode (`interface GigabitEthernet0/0.15`) and execute `ip helper-address 10.1.1.250`.
This instructs the router to intercept UDP broadcasts on ports 67/68 on that interface, insert its own interface IP into the giaddr field, and unicast the request to 10.1.1.25010.1.1.250.

Key Concept

Cisco IOS DHCP Relay Agent Interface Placement
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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