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Difficulty: MediumDHCP Architecture and Relay Services

A network administrator is deploying a multi-VLAN network on a Layer 3 switch. Workstations on VLAN 50 (10.50.0.0/2410.50.0.0/24) are failing to acquire IP addresses from a centralized DHCP server (192.168.10.10192.168.10.10). During troubleshooting, the administrator notices that the `ip helper-address 192.168.10.10` command was mistakenly configured on the physical trunk uplink interface connecting to the core router instead of the client subnet interface. Which configuration change must be implemented to allow VLAN 50 workstations to obtain DHCP leases?

  1. Apply the `ip helper-address 192.168.10.10` command directly under the VLAN 50 Switch Virtual Interface (SVI).Answer
  2. B
    Reconfigure the DHCP relay agent to forward client requests using TCP port 67 instead of UDP port 67.
  3. C
    Relocate the `ip helper-address` configuration statement to the network adapter properties of the central DHCP server.
  4. D
    Enable global broadcast routing on all physical access switchports assigned to VLAN 50.

Answer

Configure the `ip helper-address 192.168.10.10` command directly under the VLAN 50 Switch Virtual Interface (SVI).
DHCP clients issue Layer 2 broadcast requests (`255.255.255.255`) that are constrained to their local VLAN. To relay these requests across subnets to a remote DHCP server, the DHCP relay service (`ip helper-address`) must be configured on the Layer 3 interface serving as the local default gateway for that VLAN—specifically the VLAN 50 Switch Virtual Interface (SVI). Once configured on the SVI, the switch intercepts the local broadcast, sets the Gateway IP Address (`giaddr`) field to the SVI IP address, and forwards the request as a unicast packet to 192.168.10.10192.168.10.10.

Step-by-Step Solution

1
Identify the ingress interface for client broadcast traffic
Workstations on VLAN 50 send initial DHCPDISCOVER messages as Layer 2 broadcasts targeting their local default gateway interface (the VLAN 50 SVI).
DHCP broadcast messages are contained within their local broadcast domain (VLAN) and will not reach physical trunk uplinks without local SVI relay processing.
2
Evaluate relay agent placement rules
The `ip helper-address` statement must reside on the interface listening for client broadcasts.
Placing the helper address on an outbound trunk interface bypasses the client broadcast domain, preventing the router/switch from intercepting client requests.
3
Apply the correct configuration location
Configuring `ip helper-address 192.168.10.10` under `interface Vlan50` enables the relay agent to capture broadcast requests, populate the `giaddr` field with 10.50.0.110.50.0.1, and unicast the request to the central server.
This allows the remote DHCP server to select an available pool address matching the 10.50.0.0/2410.50.0.0/24 subnet.

Key Concept

DHCP Relay Agent Placement on Switch Virtual Interfaces (SVIs)
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Topics

DHCP Architecture and Relay Services

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