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

An enterprise network uses a centralized DHCP server located at IP address 172.16.10.100172.16.10.100 in VLAN 10. Clients in VLAN 20 (172.16.20.0/24172.16.20.0/24) are connected to a router via subinterface `GigabitEthernet0/0.20` (172.16.20.1/24172.16.20.1/24), while the DHCP server connects to subinterface `GigabitEthernet0/0.10` (172.16.10.1/24172.16.10.1/24). Hosts in VLAN 20 are currently unable to acquire IP addresses dynamically. Which configuration command must be applied under the `GigabitEthernet0/0.20` interface to forward DHCP broadcasts from VLAN 20 hosts to the central DHCP server?

  1. ip helper-address 172.16.10.100Answer
  2. B
    ip helper-address 172.16.10.100 under interface GigabitEthernet0/0.10
  3. C
    ip helper-address 255.255.255.255
  4. D
    ip helper-address 172.16.20.1

Answer

The command `ip helper-address 172.16.10.100` configured under interface `GigabitEthernet0/0.20` correctly enables the DHCP relay functionality.
To relay DHCP messages from hosts on a local subnet to a remote DHCP server, the `ip helper-address <DHCP-Server-IP>` command must be configured on the router interface (or subinterface) that acts as the default gateway for those clients. When configured on `GigabitEthernet0/0.20`, the router listens for incoming UDP port 67 broadcast messages from VLAN 20 clients, places its own interface IP (172.16.20.1172.16.20.1) into the `giaddr` (Gateway IP Address) field of the packet, and unicasts the request directly to the DHCP server at 172.16.10.100172.16.10.100.

Step-by-Step Solution

1
Identify the ingress interface for client DHCP broadcasts
The client gateway interface receiving broadcast DISCOVER messages is `GigabitEthernet0/0.20` (172.16.20.1/24172.16.20.1/24).
DHCP Relay functionality must be enabled on the incoming interface facing the clients so the router can intercept broadcast requests.
2
Determine the destination IP address for the relay command
The target DHCP server IP address is 172.16.10.100172.16.10.100.
The `ip helper-address` command converts UDP broadcasts into unicast traffic directed at the specific DHCP server IP address.
3
Synthesize the correct Cisco IOS interface command
Apply `ip helper-address 172.16.10.100` under `interface GigabitEthernet0/0.20`.
This sets `giaddr` to 172.16.20.1172.16.20.1 and forwards client DHCP requests across subnets to the centralized server.

Key Concept

DHCP Relay Agent Interface Placement and Configuration
Estimated Time:1m 15s
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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