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

A network administrator needs to configure a Cisco IOS router to relay DHCP broadcast requests from local clients on VLAN 10 to a centralized DHCP server located at 192.168.50.10 in a different subnet. On which interface must the administrator configure the ip helper-address 192.168.50.10 command?

  1. On the local client-facing gateway interface that receives incoming broadcast requests from VLAN 10Answer
  2. B
    On the router egress interface that forwards traffic out toward the 192.168.50.10 server subnet
  3. C
    On the global configuration mode prompt of the router rather than an interface configuration prompt
  4. D
    On all Layer 2 access switchports on the local switch to allow broadcasts to traverse subnets

Answer

The ip helper-address command must be configured on the local client-facing gateway interface that receives incoming broadcast messages.
To act as a DHCP relay agent, a Cisco router must intercept incoming DHCP broadcast messages on the local gateway interface (client-facing interface) and re-transmit them as unicast traffic to the specified DHCP server address.

Step-by-Step Solution

1
Identify the interface where client DHCP DISCOVER UDP broadcasts land.
The LAN-facing gateway interface (or subinterface/SVI) for VLAN 10 receives the broadcast frames.
DHCP clients issue requests as local broadcasts. The router must listen for and intercept these packets at the receiving gateway interface.
2
Apply the ip helper-address command pointing to the remote DHCP server IP address on that interface.
The router intercepts incoming broadcasts on VLAN 10, sets the gateway IP address (giaddr) field, and forwards the request as a unicast IP packet to 192.168.50.10.
Enabling the helper address on the incoming gateway interface activates DHCP relay agent operations specifically for that broadcast domain.

Key Concept

DHCP Relay Agent Interface Placement
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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